Something that is often ignored in this discussion is that this exclusivity only worked in the early days because the majority of players didn't reach the point of raiding. Vanilla had such a long leveling that casuals (in the meaning of putting little time in the game and not playing efficiently) didn't reach the point of "there is nothing to do except raiding". Most vanilla players did not reach level 60 in vanilla! The exclusivity worked because most excluded players didn't even know they were excluded. It's like being excluded from a party you don't even know is going on.
But over time more and more players caught up and join the party, which is why Blizzard had to make raids more accessible, because their playstyle was not compatible with hardcore raiding