Blasters are very easy to make durable if all you do is play the easiest content. You’re correct that none of these balance metrics matter if you’re playing in a setting where Blasters are durable enough. If, however, you bother to stray from that level of play, the Sentinel’s durability adds something no other Ranged AT gets.
If you are not having durability issues with Blaster, you should be playing Blaster, yes. Likewise, if you’re only playing content a Scrapper can tank, you should have zero reason to ever make a Brute or Tank. Etc etc. But the game’s balance isn’t based on how IO builds do in easy mode content, so that’s all irrelevant stuff.