Agree with most of your post except for the word "require". If they want to promote, incentivize, or reward having a taunt/DPS/control/buff-debuff set up that's fine. You could fill those roles with lots of different team make-ups as there is sufficient overlap in roles, depending on builds. But requiring any AT or powerset would be a huge mistake in my opinion.
I also disagree that being able to complete "hard mode" content with 8 blasters automatically makes it not hard. That entirely depends on the content and the blasters. If 8 random, badly SO-only slotted new players are able to do it is not the same as 8 knowledgeable veterans with bleeding edge builds doing it . . . and perhaps having a harder time doing it than if they had some support, meat shield, crowd control assistance.