One operator I know has a whole box of malfunctioning coin mechs. Yes, they wear out eventually.

If a game is being retired and still has good coin mechs, they will be removed and reused in another game or kept as spares for when other mechs fail.

I like having my games "complete" as well. Depending on the era of game, sometimes it's easy, and sometimes it's difficult to locate the proper coin box. At shows, you can usually find at least one person who has some for sale in the flea market.

When I find games, sometimes they have mechs and coinboxes, and sometimes they don't. I've also known some collectors who remove coin mechs and coin boxes from games they find and sell them separately for a few extra bucks.

If you need help identifying coin boxes, I started documenting them in an identification guide a while back:

http://www.pinwiki.com/wiki/index.php?title=Coinbox/Cashbox_Identification_Guide