This is an odd comparison since, minus Kara, the TBC raids were all split and individually smaller than many of the raids we see in WOTLK or MOP. A better metric would be to look at the number of raid bosses since that is mostly what raids are judged on.
TBC
Kara/Gruul/Mags - 13 bosses
SC/Tk - 10 bosses
ZA - 6
MH/BT - 14 bosses
SW - 6 bosses
49 unique bosses
WOTLK
Naxx/Sarth/Mal - 17 bosses
Ulduar - 14 bosses
ToC - 5 bosses
ICC/RS - 13 bosses
49 unique bosses + 3 vault bosses.
Cata
BoT/BWD/4W - 13
Firelands - 7
DS- 8
28 unique bosses + 3 vault bosses.
MoP
MsV/HoF/ToES - 16
ToT - 13
SoO - 14
43 unique bosses + 3 world bosses.
WoD
HM/BRF -17
HFC - 13
30 unique bosses + 4 world bosses.
It's really shitty to try and bolster WotLK by including Naxx tbh. Wrath had one good raid, one mediocre raid, and two bad raids (Naxx/TOGC).
I think it's fair to say that including more difficulties inevitably reduces the amount of raid content we get, but it makes it more accessible and lets especially the hard versions be significantly harder. Instead in TBC even the hard fights were a lot easier than what we get now, and MOST of it was just easy (1-2 bosses per zone that were hard, the rest a modern guild would decimate within a handful of pulls). I'd say pre-difficulty split, most of the fights fell somewhere just above current heroic difficulty.
Regarding difficulty: I mostly see complaints of "Why do I care, I've seen the boss before" from folks that are progressing in normal/heroic, not mythic. I think your average mythic raider is pretty happy with how different fights on Mythic can be. Just adding 1-2 mechanics in conjunction with tuning the rest to be challenging can VASTLY alter how a fight progresses, and as long as it's interesting and difficult we're usually happy with that. The reason this strikes me as funny, then, is because those same people playing on normal and heroic
would not see the fights at all under the old wow model from vanilla/tbc. People doing anything other than mythic are directly getting access to content that didn't used to exist, as much as "I see someone in town and I want to strive for that gear" might have been true, it doesn't actually add any content for you to consume unless you go for it. You still can. Mythic gear still looks different from shitty lfr gear and normal and heroic gear, like it's still an obvious difference. Heck, in TBC the raid gear looked like recolors of the pvp gear, but I bet you saw someone in full T6 and still could tell. If anything it's just gear visual fatigue. I know when I see new tiers pop up on MMO-Champion, even if they look cool, I no longer care all that much. We've had so much gear in this game over the years that looks unique that it doesn't matter anymore, and people are going to transmog to what they enjoy looking like anyway.
I guess to me every complaint people have about the good old days and how wow is dying generally strikes me as strawman arguments that they can prop up instead of just admitting that they're tired of the game itself and it hasn't actually gotten worse by any real measure. There are totally aspects to old wow and other old MMOs that were super fun and I think everyone enjoyed, but when it comes to still playing the game over a decade later I would hate to have to deal with a lot of that shit, and so would everyone else. At some point seeing someone in T6 while you're not running it yourself loses its novelty and you say "Hey, I want to see that stuff. Illidan is on the box and I haven't seen him once". At some point the 'community' of standing around for 2 hours trying to get into a bad Stratholme group loses its luster and you're thankful that you can be matched in to a dungeon automatically. At some point caring that other people can experience LFR or normal raiding disappears and I can just focus on the people I actually want to play the game with and what we can achieve in our mythic raid: why should it bother me that there is an easy mode in the game? I don't care when people play other games on easy, it has nothing to do with me, I care about actually enjoying the stuff that I'm doing within the game.