There is a reason most private servers buff their content. WoW gets progressively more difficult with every expansion but I think the pinnacle of raid difficulties were Tomb of Sargeras and Crucible of Storms. At the same time these are probably the worst raids in WoW history.
Having established that older raids are generally easier than newer raids there is also a weird phenomenon that even though wotlk content is 12 years old now there are still people too bad to play an encounter with 2 mechanics properly.
There are people out there who only played wotlk servers for 10 years now and they still suck at wotlk content.
Considering that hardcore raiders who want to overcome a new challenge with each new raidtier will probably keep playing retail and classic players want a more social and relaxed experience cata would probably the point were raids will get hard for the classic playerbase. But for anyone who has cleared a mythic raid in the last 4 years WoD would probably the turning point.