Archimonde doesn't really make that much sense as a final boss to Warlords of Draenor.
The Defiler has been cooling his heels in the Twisting Nether since his defeat at the battle of Mount Hyjal at the end of the Third War, and there isn't really anything to foreshadow his appearance in Hellfire Citadel other than the general "the Legion is here! Oh Crap!" vibe to the place. Gul'dan was really the best-foreshadowed final boss for Warlords of Draenor - he's the very first major lore figure we directly interact with, and the quick collapse of the Iron Horde at our hands gives us a certain degree of responsibility for Gul'dan's ascension.
One could argue that the final fight in HFC is essentially Gul'dan pulling off a far more successful trick than old Wilfred Fizzlebang (RIP) in Trial of the Crusader. Gul'dan had already rebuilt Mannoroth's body to pit us against the Destructor, and because even that wasn't enough to stop us, he brings forth Archimonde.
But it's still, honestly, pretty anticlimactic. When he was defeated in WCIII (and the Caverns of Time, which we can, I think, safely just interpret as the same exact event but in a different game format - it's one of the few CoT instances with no Infinites in it) we assumed he was dead for good, but the "soul stays in the Nether" type of resurrection that we knew Dreadlords could do has been extended to all demons in Warcraft lore - which also means that this Archimonde is, unlike the many characters we've seen in Draenor, one and the same Archimonde we faced before.
The consequence of this is that when we beat Archimonde in HFC, there's basically nothing to celebrate. Yes, Draenor is safe for now, and the Legion's claim on the world has been rebuffed (and we've all just kind of decided to forgive Grommash for some reason - and even let him be the one leading the cheer,) but there's no sense of relief that Archimonde the Defiler has finally been killed, because he hasn't, really.
Resurrection in fiction is always a tricky thing to pull off. It can be a really cool plot development, but overuse can make a world's stakes feel lower. For example, in a Song of Ice and Fire (the books Game of Thrones is based on,) there is certainly the magic needed to bring someone back from the dead, but the instances of this working have been nothing short of miraculous (meaning rare and unprecedented) and also leave the subject of the resurrection deeply disturbed by the process - suggesting that this return to life is not necessarily a good thing.
On the flipside, however, Blizzard killed off some very major characters in their first few expansions. Both Illidan Stormrage and Arthas Menethil were very popular figures, and Deathwing was a major presence in the game, even if we didn't see him until Cataclysm. We lost a bunch of big Warcraft names, and now I sort of wonder if Blizzard now wishes it had spared them somehow.
Clearly they do for Illidan, as the Betrayer (man, so many of these guys have a "the blank" epithet, though to be fair, some share them, like Deathwing and Sargeras the Destroyers) will be coming back to life in Legion. Indeed, I highly suspect that Illidan will wind up being not a villain, but a champion (albeit a bitter, angry, and volatile one) in the fight against the Legion.
When you write a limited story - even if it's a long series of novels - you can reach the end of a story arc, and sometimes that means that the character at the center of that arc needs to die. Wrath of the Lich King sent of Arthas fantastically. We closed out his story, and you couldn't really ask for a better conclusion to his arc (ok, I'm sure you could think of somewhat better ways to send him off, but the point is, this was a satisfying one.)
The problem, though, is that now you can't have Arthas in the Warcraft Universe anymore. Arthas is one of the most iconic villains of Warcraft (possibly the most iconic,) but he's out, and there's really no logical way to bring him back (ok, yes, he was undead, so maybe you could - the point is that doing so would undo everything about his death in Wrath that was so great.)
Perhaps when they first started making WoW, and even in the first few expansions, they didn't really expect it to have the kind of longevity it has had. By now, of course, the thinking at Blizzard has shifted toward WoW as an ongoing saga - analogous, I think, to a comic book series.
There's a reason why Batman has a no-killing policy. Well, two reasons, with the first being that we can always root for him. But the other reason, more important to this post, is that that means all the memorable characters can actually stick around, even if Batman wins against them. The Joker can lose, but still remain a presence in the Batman universe (ok, DC Universe, but you know what I mean.)
Christopher Nolan's Dark Knight trilogy version of Batman had the freedom to kill off villains (though interestingly, not the Joker) because they knew it was self-contained - it was never going to be more than the three movies it was, and so having Two Face die only briefly after becoming a bad guy didn't really hurt them that much; they wouldn't have even really had time to do more with him in the third movie anyway - the point of his character was made, and he was free to get killed off. But the way the comics work, characters can only be killed off on a temporary basis.
It's a tricky balance to strike. Arguably, we defeated Archimonde Joker-style (though actually even less definitively, as we have no Arkham Asylum for demon souls.) I still think that he wasn't really a great choice for a final boss because he really didn't have any presence in the expansion up until that point. I'm glad they didn't go with Grommash, as that would have felt too similar to Garrosh in Mists, and I suppose they decided against Gul'dan because, though he's a very different kind of orc than Garrosh, he's still an Orc, and also that they wanted Gul'dan alive to set up Legion. But unlike the Joker, Gul'dan's going to get killed off (again-ish) early in the expansion, so rather than keeping him around as a constant thorn in our side (/threat to our planet,) instead we'll just see him die as the fairly epic final boss of tier 19, rather than the majorly epic final boss of a whole expansion. It's threatening to get into Alias territory, where instead of cliffhangers, they just had what should have been the climax of each episode simply happen at the beginning of the next one (to be fair, I've only seen the first two or three episodes of Alias.)
In a way, I think Garrosh was, until those final quests in Nagrand, a good example of how to sustain a villain. We definitely beat Garrosh - he was on the floor, and later dragged off in chains - but he was still alive, and still a threat. But the next time we saw him in person, Thrall killed him, which means that we lost Garrosh as a figure within the universe (multiverse,) just like Arthas. Gul'dan will at least get to last past Legion's level-up experience (well, to be fair, Garrosh was kind of the final boss of the level-up experience in Warlords - most of my characters have dinged 100 either in the middle of that scenario or by turning in the quest,) but believe me, by the time we're thinking about 8.0, we'll have kind of vague memories that Gul'dan was a thing in Legion.
I think that ultimately, Blizzard needs to start working on some new villains and seeding them now within the lore. Garrosh was actually a great example of this, even if some could reasonably argue that the direction his character went was disappointing. It's dangerous to put too much weight on a figure who has just been introduced (though you can pull it off - Lei Shen, the Thunder King, felt reasonably epic as a mid-tier boss,) but you'll also find yourself having to constantly resurrect and re-resurrect lore figures to use as bosses if you can't come up with new characters.
This all makes me very curious to see who the final boss of Legion will be. I really, really hope Blizzard knows already, so that they can build the expansion as a big lead-up to that confrontation. But they also should try to give themselves some wiggle room. The Burning Legion is arguably the big bad of the entire Warcraft universe, so there's naturally going to be a big question of "where do we go from here?" once Legion reaches its conclusion.