1) The majority of the playerbase play Rust/have played Rust before to my knowledge, everyone I've spoken to who have played rust and mono have said that they would play a monolith rust server.
2) Gmod is dying, monolith Gmod is dying/dead at this point and tbh it doesn't look like it will be coming back, even if it does I don't see the harm in someone in mono staff looking into something like this, you could always run both, Rust requires little staffing when compared to Gmod and yes it would require someone to look after it, I'm sure that would be possible though.
3) As for losing money off the cost of servers for this type of thing, Rust is perfect for this, you could easily have a VIP type thing, with stuff like skinbox, furnace sorter, and other helpful (but not p2w) plugins. This way you could run it without having it be p2w and also not lose money off it.
4) Basically, whether gmod monolith dies or gets revived, someone in mono staff looking into something like this can't hurt.