I agree with you to a certain degree but let us be realistic and not expect Anet to completely overhaul all boons and support classes.
At the time being, I don't think that the issue lies with the need for specific builds in order to have specific buffs. By EoD release, we will have 3-4 classes being able to give quickness and 3 classes giving alac. The problem here is with boon support builds being unable to buff the whole raid. This is why you see too many "slaves" in the raid trying to compensate for it.
A small fix that can reduce the number for "boon slaves" and "mandatory meta classes" is to make all boon builds being able to buff the whole raid (10 players) instead of buffing 5 players only. This will improve build diversity and reduce the need for having 2 quickness slaves for example.
Another fix that can remove the mandatoriness of a banner slave warrior is to standardise banners with buffs like Assassin's Presence, Spotter, Pinpoint Distribution, Empower Allies,.. and give classes like ranger, revenant, engi,.. the ability to replace warrior's banners.