PMG

Constant hot environment loses the magnetism with time.

Second, rotating magnetic field of generator also induces eddy currents in side the magnet body, that also affects the magnetism of a permanant magnet.

magnetism is only destroy with heat if any permanent magnet put in to fire .it will loose its property

My experience is that PMG loss is not responsible for lack of excitation- my experience shows s/c windings- this is @ remagnetizing PM(which weren't demag any way)imho- this is a subject where one can't get relevant info as no industry as such exists- thowaway- buy new!.

the permanent magnet lost its magnetism.There was no source of heat etc..

Are you sure was a permanent magnet system?. The ones i have seen are iron only which magnetise & provide excitation thru self induced flux as gen turns- when stopped iron cannot retain magnetism- fault occurs as I have said when s/c turns reduces inductance & therefore self magnetism- I have obtained further life out of gen by using batt to excite rotor(where possible).