Given that plasticly deforming aluminium takes away from its fatigue life, repeatedly realigning a replaceable aluminium hanger to compensate for derailleur wear is not a good idea. Work hardening is a real thing. When I was a bike mechanic, the rule of thumb was that realigning a crashed aluminium hanger once would probably be fine but doing it more often risked failure down the road.
this is a very sensible precaution. However I note that in the last couple of decades, the range and quality of ductile aluminium casting alloys has improved somewhat. I therefore fancy my chances of straightening a bent replaceable hanger a bit more these days.
In general terms a ductile material will plastically deform (and it may work-harden) up to a point and then you are on 'a slippery slope' to failure.
The curve above is from tensile testing but something similar may be seen in bending too.
So when you are straightening something, if the force required to move it is still increasing, you probably have a way to go yet. However if it isn't or it is actually starting to decrease, then you are in the poop and the part is well on the way to failure.
A really ductile material will withstand several tens of percent elongation to failure; this might correspond with straightening a bent hanger a couple of times (depending on how bad the bend was and how ductile the alloy is) or resetting a gear hanger (by a much smaller amount) about a dozen times or so.
cheers