I can't imagine that any class would come close to Warlock DPS in raids if they were allowed to roll dots while shadowbolting.
People need to accept that raiding was poorly designed compared to the rest of WoW, in Vanilla. Not just the debuff limit, but the overall massive healthpools, dps spam, even down to having 40-players together was simply not complementary to the design of most classes in WoW.
Instead of changing the content (i.e. raids) Blizzard decided to change the entire rest of the game (homogenize classes, unlock shamans and paladins from faction, bizarre specialization system that now exists, no more hybrids, no more buff classes (no more buffs at all, actually), etc, etc.).
This is why Vanilla fans are completely dissatisfied with retail. Vanilla fans saw raiding as an excuse to keep playing WoW. Retail fans, and fans of retail changes see WoW as a game where the point is to raid. That is really all WoW has to offer at this point. It is nothing but a series of raid encounters.
Vanilla didn't have good raiding. Vanilla had good everything else. Debuff limit should not be changed. You'd have to change a hell of a lot more than the debuff limit to make raiding good in Vanilla.