I’m not at all sure I’m in the target audience for this question, but I’m not sure I’m not, so.
I quit partway through Cataclysm. I regret that I didn’t quit earlier, when the dungeon queue was first introduced. In Classic and in BC, forming dungeon groups required social interaction, and once we’d formed a group, we were all pretty committed to finishing the dungeon. There were occasional jerks, but the worst person I ever encountered in those days was still nicer than the average person once the dungeon queue was introduced.
The dungeon queue, combined with the ease of WotLK dungeons and that stupid championing system, changed all of that. Suddenly, instead of coming up with groups for the dungeons we wanted to go to, we were expected to go to seven random dungeons a week, with strangers who increasingly, very fast, acted like everyone there was a bot. Within a couple months, just saying “Hello” got treated like it was bizarre…or, more, like a strange bug. Wiping on a fight invariably resulted in the group picking a damage-dealer and kicking them; needing to go afk to go to the bathroom resulted in getting kicked except for the tank. (If the tank did it, the rest of the group would keep running ahead without them until they wiped and then flip out about the tank not being there, no matter what the tank had said; who was paying attention to chat?) The only approach to dungeons quickly became “run ahead at maximum speed.”
I said “expected to” earlier. You could say that going to dungeons was optional…but, increasingly, the game was about getting better purple pixels. (On the subject of purple pixels, WotLK was the first expansion where epic-level gear dropped from bosses in any normal dungeon, not counting the tiny chance epic-level gear always had of dropping anywhere.) There was no solo fun content, just super-rigid quest chains and daily quests that were designed to be tedious, not fun. All the development effort was put into raids.
WotLK also revised the talent system in a way that annoyed me significantly at the time, but should have bothered me much more as a sign of things to come. BC deliberately introduced substantially more class synergy; for example, enhancement shaman were given Unleashed Rage, which buffed all physical damage-dealers near the shaman when they scored a critical hit. Shadow priests were given Vampiric Touch, letting them restore mana as well as health to their groups.
WotLK went massively in the opposite direction. Stormstrike’s “increase all Nature damage to the target” debuff was changed to “increase Nature damage to the target from the shaman.” Shadow Weaving was similarly changed to only buff the shadow damage of the priest with that talent. Vampiric Touch was changed to grant “Replenishment,” a means of restoring mana which was common to shadow priests, survival hunters, and retribution paladins, and the devs explicitly said they meant for groups to need someone who granted Replenishment–not for any particular class or spec to benefit from the specific presence of a shadow priest. Class synergy in general was deliberately all but eradicated. WotLK was also the era that introduced the concept of the content drought, because it was the first expansion to assume that all players would be raiding and “new fun content” could be safely equated with “new raid content.” Subscriptions peaked and started to decline. Large numbers of people also started unsubscribing temporarily “until the next raid comes out,” for the first time in WoW’s history. Sane developers might have recognized these as warning flags and tried to reverse course.
Then in Cataclysm, they slashed the talent system in half, forced everyone to put a mandatory number of points in one tree, and replaced most of the quests with rigid quest chains designed by someone who thought he was hilarious and very rarely managed to achieve “funny.” “Blood elf men are gay lol” became a theme. If there was a quest your character wouldn’t do, that meant you were done in that zone, since every zone’s quests were an absolutely rigid chain. The “good” news is that doing an entire zone’s quests meant far out-leveling that zone. Also at this point, every major patch entirely revamped the game’s mechanics; it became clear that Blizzard had no idea what they were doing. This was also the era where tier set bonuses started explicitly boosting talents that weren’t necessarily in your spec, because Blizzard had explicitly started taking the assumption by a handful of the highest-level raiding guilds that “everyone uses exactly this build” as gospel–shafting the majority of the original players who simply rolled our eyes and ignored those guilds’ declarations.
Then Mop came out, got rid of the talent system entirely; now everyone who played (e.g.) a fury warrior truly had the exact same build as every other fury warrior. It retconned Varian Wrynn, the frothing fascist who had screamed he was going to cleanse Azeroth of the green-skinned aberrations, into a pure-hearted hero who fought the Horde only because he had no choice, retconned Garrosh (who admittedly had a new personality each expansion up to that point and none of them were particularly appealing) into a frothing fascist, and explicitly declared the Alliance to be about “lawful good overdrive.” Those are the things I can definitely know, as someone who never actually played it.
In a bid to entice me back, Blizzard gave me two free weeks of game time in late 2018. I logged in and played enough to confirm that there are now four classes: tank, healer, ranged damage, melee damage. The only differences beyond that level are purely cosmetic; a Marksmanship hunter plays just like an any-spec mage. Also, the plot of BFA is: Another war has broken out between the pure good Alliance and the pure evil Horde, with non-evil Horde members trying to help the Alliance. Just like Mop. The dev’s efforts to differentiate BFA from Mop have been nothing short of laughable.
So, to answer your other question…could they change all the things I hate about Modern? Someone could, theoretically; the current devs never could. Even if they revised the mechanics to make a fun game again, convinced me that they weren’t going to yank the ground out from under me in the next big patch, and somehow deaIt with the fact that the vast majority of the playerbase who’s still there would be freaking out about the very concept of having to communicate in dungeons and deal with 36 distinct types of characters instead of 4, I don’t know how they’d be able to repair the damage they’ve done to the plot without throwing massive amounts of “it was all a dream”-level retcons at it.
But could they change what I hate about Modern? Practically speaking, no, and so there’s no real answer to “if they did, would I play Modern and not Classic”?
Okay, this was long, and an answer from someone you hadn’t truly asked for one since I already quit long ago. I hope something in it is useful to someone.