Ingenuity is mainly a proof-of-concept to the main Perseverence mission. It worked very well considering the challanges. When the helicopter is done being "useful", will it be stress-tested with i.e. faster/longer flights, more difficult terrain, etc to see how much it can take before crashing?
In some sense, it already is.
Ingenuity was designed for 5 short test hops during the first 30 days. Pretty much every flight since then was higher, farther, and longer than originally designed and planned, in addition to the helicopter only being planned to operate for 5 flights and only for 30 days.
When the helicopter is done being "useful"
It was actually never planned to be "useful". Even the scouting missions it has done recently are already outside of its design criteria. So, the "useful" flights, as you call them, already are stress testing in some sense.