Sentries are still fragile as heck, but at least this way they do mostly only shoot things they can damage. The targets you need to keep away from them you can also at least spot in a crowd (chargers, titans) and deal with quick or accept that they'll be a problem that gets distracted. Hiveguards are too numerous for that.

I'd honestly accept them being tankier if shot in the head and other armoured parts though, if it didn't make sentries so wasteful. A system where lower pen still can damage through higher AVs but with even further reduced damage for example would be fine. But that's not what we have.

Also if they did just guard nests like the name implies instead of showing up in groups in bug breaches or called by brood commanders.

There's also a question of faction identity.

The way I see it: Bots are the precision targets. Illuminate have damage mitigating equipment and combined arms. Bugs are the hordes you mow down. Feels odd to have a heavy pen chaff unit in there.