I've asked myself the same question a bunch of times over the years, but everyone (and I do mean everyone I've met) who dumped their whole collection regrets it. Part of it is that you end up getting so much, that you start feeling guilty. It's like you start thinking 'hey, I have all this stuff I never use, and it's taking up all this space...' That kind of thing.
But you know what? Who cares? So what if you've spent money on it, and so what if you don't use it all the time. I go six months without even going in my game room at times. But I always come back. When I walk in there, sit in my comfy chair with an ice cold beer, and a plate of something tasty, surrounded by hundreds of games that I love, it's something special. That room is like a sanctuary. No annoying job, no stress, no worries, no crap. Just my own little private library of alternate worlds that I can plug into. Sure, there's no way I can realistically play it all at this point, but so what? Maybe I want to play some Ys or Salamander, or perhaps a few rounds of Samurai Showdown on the neo. Maybe it's time for some 2600 fun, or a strategy title on the Intellivision. Or maybe a few white knuckle rounds on my Defender arcade cab. Or... anything.
The point is, it's there, when I want it, and it's all ready to go. I built that collection over half my life. No way I'm getting rid of it.
That said, I do downsize from time to time. Here's the thing. If you want to downsize, get rid of stuff that you never play, don't care about, and only sell stuff that is easily replaced. *Always* keep rare stuff you will not be able to get again with out a lot of money/effort. For example, I have a respectable pile of Vic-20 & C64 stuff. I don't really care about it. Never play it either. I may end up selling all of that at some point, but I will use the money to get a few Neo or PCE games.
Never, ever, EVER sell off the whole thing. Being tired of it is ok, I get tired of it yearly. I have lots of other interests. But that 'being tired' is a mood that will ALWAYS pass, and when it does, you'll have the game room waiting. Just my two cents.