As a hobby, I don't think original arcade game collecting has or will change much.
People will come, people will go. It's a small niche to begin with.
At least in my area, prices have taken a slide in recent years to the point that the regular semi-annual auctions have ceased.
I hear plenty of people say that there was a bubble years ago and prices are getting back down to what they should be anyway.

That core group of people who have been around for a long time aren't going anywhere.
Although linked, I consider the arcade hobby and the arcade emulator hobby two different things. I've owned a couple games in the past, but didn't know how to work on them and don't consider myself part of the arcade scene. Building stuff and getting the games working with emulators is my hobby.
I don't actually play the games much.
I keep telling myself that one day I'll be done and just enjoy the games, but part of me considers time spent playing games lost time with nothing to show for it. It's ok to do for a half hour here and there, but I'm not spending much of my free time doing it. Same goes for movies & tv. If working on something, I'm learning or improving skills.