> When the engine is passed of the test cell there is a declared "stall margin" which is basically how much wear is allowed until the engine becomes "worn out"<
Partial credit answer
Stall margin is actually the difference between the operating line and the stall line, both of which vary throughout the operating range of rotor speeds and airflow.
The stall margin can and does vary with wear (deteriorates) but also with altitude, temp and the accell or decell rate of the rotor (bodie stalls etc.)
Some recent examples of this have been the altitude stalls on the JT9D where the stall margin decreased due to combinations of altitude and wear (somewhat of a nuisance problem allowed by maintenance until the pilots squwaked it)
Also some recent problems with even young engines have been aggravated by temp changes and bleed loads rather than significant wear problems.
Stalls/surges cause temporary bowed rotors and aggravate wear and in some engines can even cause the blades and vanes to touch (not good) which significantly lowers the stall margin even further.