From Ghostbusters? He wasn't.
From being a regular on-screen presence? Yeah, he disappeared into a drawer 10 years ago, and for the most part only comes out infrequently to voice or portray Ray. I would hazard the guess that most people under the age of 20 would not generally recognize him.
Probably safe to presume the same for many characters when it comes to sequels. You either know who it is or if you are not a fan, you don't?
Their other work is irrelevant unless a movie was just going for star power. Celebrity star power is a different discussion altogether. It is funny you are so cynical about it when it would actually mean more that someone was cast for their character portrayal and not popularity. If people under the age of 20 have seen the other Ghostbusters films, they will. As he has been in each one.
Mothballed-As a verb, "mothball" has a metaphorical usage, meaning "to stop work on an idea, plan, or job, but leaving it in such a way that work can continue in the future". " Mothballed" is a common adjective to describe ships or aircraft which are stored for long periods, but not sent for scrapping.
The dude never retired from acting or entertaining.
Phrase does not really apply here with your ten-years mothball situation. As there was never ten years of ghostbusters without him or his character.