Many decisions have been made all the way across the Sentinel AT in the name of balance, to prevent the archetype from being too strong or treading on the feet of other ATs. That's fair. The whole point is that a fan-created AT shouldn't be too stupidly strong and overshadow the older official live ATs. But it does mean the Sentinel's being designed and balanced from a very cautious perspective.
There's comments in this thread that argue Sentinels aren't, and don't need to be, as effective as other ATs in endgame content - whatever you wanna define endgame content to be, and they're just fine as easy-to-play solo-friendly characters for levelling. But that's the kind of thinking which leads to... I mean, this has dropped off with balance changes, but you know how people used to say that Masterminds had no place in Incarnate Trial content, and shouldn't be played in Incarnate Trials? Yeah. That's not good thinking.
That's the kind of thinking that ends up with teams that are exclusively resistance armor Brutes and Cold Corruptors.
Ideally everything should be viable. There's comments in this thread saying there's already been a Sentinel pass, and limited volunteer dev time shouldn't be spent too heavily on this one fan-created AT. That's a reasonable practical comment - there are other things in this game that need fixing. But in principle we want everything to, y'know, not suck.
The big question here is what the niche or role of a Sentinel is supposed to be. It's not just "ranged DPS that's tougher than a Blaster", or "Scrapper with more range". The vulnerability inherent positions them as a debuffer. The inherent also gives Sentinels increased perception and resistance to per debuffs, so that's supposed to be a role as well.
The issue is that the value of vulnerability is hard capped, since we can't stack 'em on the same target. This also means there's a limit on the number of Sentinels it makes sense to bring on a team, especially if you're mainly concerned with an AV/GM fight... perhaps the only AT where the game outright tells you that bringing multiple of the AT is a bad idea. So the "Sentinels are debuffers" thing is a wash.
And let's face it, does anyone care that Sentinels have better perception? It doesn't come up that much in PvE. Is it valuable in PvP? Hell if I know. The fact that the perception thing hasn't really been brought up in this thread demonstrates that it ain't factoring into how people think of the Sentinel, even though it's supposed to be part of the role identity.
But what's the solution? Should any further adjustments to the Sentinel change the role identity? If they become ranged tanks proper, as the OP suggests, that's a completely different role. Or is the solution figuring out some way to make 'em better at their current role?