Problem is in Vanilla you could feign it was dev ignorance, they assumed with like 8 tanks it wouldn't be so bad but they also were trying the hard hitting way which... totally didn't add to that thinking - I mean you had x amount of healers or damagers that if died could maybe see the end of the boss so tanks were part of that ideal just... totally didn't get there in gameplay.
Its fun though that now it's generally a mechanic that kills a tank if not executed properly but there is still a margin of error to be had, so like a boss does a stacking debuff that kills you with more damage or instant slaps you, kinda thing and has been like that since earlier like TBC. There are always going to be bosses that ignore that rule but I would say in TBC they were doing a mixture of being nicer to groups and using it as a mechanic.