To be honest I don't think raiding has only gotten slightly more difficult over time. I can't tell you how many fights I've seen on retail where you can stand in the fire and not die.... even on simple stuff in MC like Gehennas rain of fire.... you would die if you didn't move. Plus the players today have cooldowns to survive that kind of stuff. The only cooldowns back then were Bubbles for pallies, Iceblock for mages (which required you to spec frost), and Shield Wall which was on a 30 min CD. Compare that to retail where each class has tons of survival cooldowns, not only that but some classes can give other classes their survival CD's, and there are AoE related survival cooldowns too lol.
I was a heroic raider back in MOP, had 12 Ra-den kills and I can honestly say, most of the bosses were pretty easy, especially once you had gear. The same held true for vanilla, while you are progressing in a place like AQ40 though, you can still easily die to shit even once you are close to getting that BIS set tho. That being said, a few of the fights were legit difficult (heroic Lei Shen for example) but if you are in a good guild, know your class well, and work on your gear, you can see mythic raids easy. Ditto with stuff like vanilla and BC. But just because Molten Core was easy, doesn't mean the rest of the raids were simple too.
Plus you had to do stuff to increase your chances of success outside of the raid, mainly farming consumables and getting buffs ahead of time. Nowadays you just show up to the raid and that's it lol.
As for people loving Wotlk yes, there are some out there, similarly there are many like myself that thought it stunk. Nobody has asked for Cata servers though, so your point about people "hating expansions and loving them later" is moot. People like vanilla and BC because they are more fun then what retail has to offer. Nobody is under the pretense of them being perfect, but they are 10 times better than WoD.