They have that, it's called LFR.
And 10m vanilla raids were just slightly larger dungeons. In fact they've been reformatted to dungeons now...
Exactly. My guild cleared MSV after about a month of raiding, but we'd only raided every Sunday. We'd have progressed further in if scheduling didn't wipe our raid out completely (everyone was either in college or raising kids at that point).
Casual never meant less skill, it just meant not playing the game 50+hours straight...which is what old Vanilla content practically demanded of you, half of which was spent spamming for groups to do long, boring content.