But even if your companions are replaced by ones of greater skill as you advance the story, it wouldn't give you more choices. To stick with your Dragon Age example, you could get Blackwall at the beginning and he'd be replaced later on with Iron Bull. Or vice versa. How does that help you? It's still going to be completely subjective which character you personally prefer, personality-wise and as a team component. Bioware can't adjust that beforehand as, for instance, Blackwall is only preferable if you're not playing a one-hand+shield character yourself.
I see what you are saying and for the most part it makes sense. The only problem with this is that the devs can incorporate this into the process. So lets say you start out on levels 1-5 with a certain character. Maybe this character has flaws that makes them suited better for other things other than battle. Maybe they find their place as a researcher and decide to contribute to your team another way. That's how I see it anyways. Besides that, yes, the change from one group of companions to another is mostly superficial and would only be there to show that the PC is somehow supirior in some way to the people who he use to work with.
Also, Mass Effect is quite different from Dragon Age in this regard. You don't have to rely on filling certain roles with the 'Tank, DPS, Support' trinity you refer to. Unless they mix things up a lot with ME:Andromeda, you're perfectly fine to bring whoever you like the best, even on the hardest difficulties 
You are correct. ME is, in a lot of ways, completely different than DA. I guess the only thing left to ask is whether it would be optimal to have, say, 3 sentinels instead of a otherwise balanced party if the game were different. I fully admit I don't know much about ME, but I know I like BW so that is why this new ME game is going to be one that I give a fair shake.