A limo is unique in that in kind of has to be a reasonably new model year to be useful. Even if its physically sound and clean, who wants to rent/own/be seen in an old limo.
So what happens to them? Who buys them (I know they go pretty cheap because of, well, what I said above). Do they just get sold for scrap & parts to junkyards?
I’ve wondered the same thing. The custom stretch models became popular in the eatly '80s, but you never see an '83 10-seat Caddy out on the streets the way you do see old cabs and cop cars.
It might be that the radical surgery they undergo early in life does not contribute to longevity. But I don’t know that. Are they prone to certain mechanical or structural problems?
Just tonight, though, I had to wait for a prescription at the drugstore and flipped through something called the Dupont Registry where I noticed a 10-seat Cadillac Escalade with 36K miles being offered for $87K. That’s a bit steeper than what you might pay for an old cop car. Not nearly as practical, either.
A fire company I know of picked up an old Fleetwood limo and turned it into a Chief’s car. Plenty of power, heavy-duty everything-it served them well. Also kinda cool to see a black Caddy limo with red lights on the roof. 
Lower end airport service, for one. I can personally vouch for some early 90’s limos being used for this purpose at Newark Airport. Crummy but cheap.