The game is easier than it has ever been. What I think a lot of you have been neglecting is the fact that in addition to boss mechanics being (generally) simpler in Vanilla/TBC the classes themselves were far less mechanically capable. In Vanilla simple mechanics like raid-wide damage were difficult to deal with, and a boss like Ra-den would be utterly impossible because tanks barely had any forms of damage mitigation.
Now, with all the guides, easy gear upgrades, etc. it would stand to reason that classic Naxxramas and Sunwell pre-nerf were harder than nearly any raid since.