This coming Tuesday, Legion will be getting its eleventh 5-player dungeon, meaning that an expansion that already surpassed the past two in 5-player content might bring us closer to the numbers we saw in the first three expansions.
But the dungeon system itself has gotten significantly more complicated in Legion. If you only run dungeons through the LFG tool - specifically the version that puts your group together - there's really only practically eight dungeons, the same as there were in Warlords.
The Arcway and Court of Stars, which reward the same level of gear as other Mythics but are, I'd contend, a little harder (not in strictly numerical terms but mainly navigating them and making sure you pull the trash carefully,) are both locked behind a pretty substantial quest chain and reputation grind. With this upcoming patch, however, your alts will be able to run these dungeons as long as you've completed them on at least one character, regardless of where they are in those quests.
On top of this, there's the Mythic Plus mode, which is a sort of evolution of the Challenge Mode introduced in Mists of Pandaria, except that it's less about bragging rights and transmog gear and instead conceived as a truly hardcore alternative to raiding.
The Mythic Plus mode is also the way that Blizzard planned to allow the creation of new dungeons without automatically supplanting the old ones (anyone who played during 4.1's Rise of the Zandalari can attest to how damaging it was for there to be basically only two dungeons worth doing.) New dungeons in Legion would actually have the same rewards as old ones, but with Mythic + they could all get arbitrarily more challenging and proportionately rewarding.
The Karazhan dungeon... doesn't really follow that model. Karazhan will be a Mythic-only like Arcway and Court of Stars, but its Mythic (no plus) is expected to be harder and is more rewarding than other Mythic dungeons (with 855 and 860 rewards.)
Now, perhaps we should consider Karazhan to be sort of its own thing - they seem to be conceiving it much more as a 5-player raid, with a whopping eight bosses (and I'd expect a ninth bonus boss that is probably Nightbane, just like the original raid.) Blizzard is encouraging guilds to break up Karazhan runs into different nights much like you'd do with a raid (that you don't have on farm mode.) Given its length, I don't even know if there is a Mythic Plus version of Karazhan, as that would presumably require a tense marathon session (Bronze: three hours!)
If Blizzard wants to add more dungeons after Karazhan (and I really hope they do,) I'd expect they'll be more conventional 3-5 boss dungeons. While I don't know that these will need to scale all the way down to the leveling process (frankly, I don't even often fit Violet Hold into leveling,) it would be nice to have more options for level-cap Normals and Heroics.
On the other hand, is Mythic Plus really the long-term solution? Let's say we're in patch 7.3. We're bored with killing Gul'dan over and over in the Nighthold and even LFR-only players are rocking an average item level of 860 or so. If they release a new dungeon, are we really going to want to hope that we get a keystone for it and then have to do a timed run of it (no bathroom breaks!) if we want to get any actual gear upgrades?
Mythic Plus may be good for keeping older dungeons relevant, but I suspect that Blizzard anticipated this issue at it is why we're seeing Karazhan coming out with higher gear rewards. Right now, I think most players are going to be decently happy to see 840 pieces, but that's not going to be the case forever, especially as we get into later raid tiers.
In fact, especially given that Mythics require player-built groups and I'm already seeing regular Mythic dungeon runs requiring a higher gear score than the actual gear that drops out of the place, I think it might behoove Blizzard to make Heroic and lower difficulties of newer dungeons more appealing.
Still, whatever solution they come up with, I hope that they aren't discouraged from adding more 5-player content as the expansion goes on. We still don't really have any idea what 7.2 will bring (Nighthold is, theoretically, coming before then - though of course we thought that it would come before 7.1. Not that I'm complaining - it's more that 7.1 is coming early than Nighthold coming late.)