I'mma just point this out:
I was recently running some alts through dungeons for RaF leveling. I found the quality of say UBRS or Strat (in terms of texture, design, and overal 'Look and Feel') to be vastly superior to most of the Auchindoun/Hellfire dungeons. Some dungeons in TBC are better than others (specifically the ones geared to max level players. The Tempest keep dungeons, Shattered halls, Shadow Labs, etc.)
If you compare those, side by side, to WLK dungeons, once again, there's a huge disparity in quality. Utgarde keep vs Mana tombs? UTK wins it, hands down. Nexus is gawddamn gorgeous compared to, say, Steamvaults.
Overall, we're getting fewer dungeons that are made to a significantly higher standard.
MoP had fewer dungeons, but they have a significantly higher standard in terms of textures clipping, texture resolution, spectral mapping.
I don't think they can really change into a lower gear, in terms of graphical requirements per dungeon, because it'll stand out (as the rest of the game has also "Stepped it up" in a similar nature.) and scenarios were a decent option because they use existing locations and are easier to build and balance.
They did mention they were considering adding more, and also adding additional difficulties of existing ones so we won't have Heroic-Zandalari-Grind-mark-2.I'm relatively certain that they're talking about the amount at launch. There will most likely be more added in later patches.