There are two views on value- one is film will die and in 20 years time you may not be able to get rolls of film to put in it thereby reducing its value etc etc. There is however a counter argument that would say Polaroid died out but got revived, Fully Fashioned Silk Stockings made a comeback even though the machinery that made them was broken up, vinyl records are making a comeback. Its not impossible for someone to make 35mm film (its certainly easier than making Polaroid or the machinery that produced silk stockings). Given scenario 2 that it doesn't die the number of cameras available to work with will diminish, some will break down, some will be destroyed, some will be broken up for parts - there will be attrition. As a result some of these will go up in value (maybe). It will be a long time though.
The FM2 was a beauty though and doubtless you would find a buyer BUT.....
In a nutshell if it has any sentimental value I would keep it. Treat it with care, have it serviced, hope it goes up. I still have my dad old Agfa from 1958. Almost all of our family photos were taken with it and it still turns out good results. Not bad for a 60 year old camera.
I sold a lot of gear back in the day when money was tight. Was I really that desperate for a few $$$ - probably not. Do I regret selling it - yes and yes and YES ! My inclination would be keep it but only you can make the call.