That seems more like a Touch Attack mechanic. In D&D (and presumably other systems but I don't know of one off the top of my head) there's this mechanic where ghostly attacks, and some magical attacks, just need to touch you - they're not really of this world, so they ignore physical defenses. Your Dexterity bonus still applies to see if you can prevent being touched (Dodge, here, obviously) but your physical defenses don't mean much when a ghostly wraith touches you and the chill of the grave saps your strength. Doppelgangers seem like the ghosts of the UnderRail game world.
I like the idea of dispersing the doppelganger when the creator dies, yeah. But as much as doppelgangers are a pain when you don't have psi, they're the opposite of Psicognitive Interruption/Black Dragon Poison, which is only a pain if you do. Seems fair to have a weakness tailored to each build approach, though I do mostly play psi so the doppelgangers don't bother me much.
All Thought Control skills seem to directly affect the target's mind, though, and then there's that one researcher in Caerus Residential Block who
went all Event Horizon on himself, apparently due to a doppelganger fucking him up from within his own mind. We do see legitimate
psionic ghosts down in Deep Caverns too, but lore-wise they seem to be a different thing than doppelgangers, despite using the same gameplay mechanics with some exceptions (for one, they don't have true-sight).
Anyone with at least 60 Biology can craft and use a Psi Reinvigorator and then a Psi Booster to clear the Inhibited status and restore 75 psi points, so there's an effective countermeasure that's not exactly costly in terms of required skill points. For those without psionics you can only try to run away, tank the damage (not really an option on Hard and above if there are other enemies, except maybe if you have 10+ CON) or rely on Dodge, which does very likely need heavy investment to be reliable. Yes, they are build-specific weaknesses which naturally should exist, but while you can outright eliminate the problem of being Inhibited, the damage from a doppelganger you can only attempt to mitigate. IMHO, that's not a small difference.