I may be one of the few who can see a finite end to his "collection." I've always had modest aspirations, and the current state of my rifle collection shows it: 8 rifles in hand, plus one more on order. I can see maybe one or two more after that, but that should be it.
One reason for this is that I have no plans to acquire anything that would involve a new caliber. As it is, I own rifles in .22 LR, 9mm, .357 mag, .223, .308, 7.62x39 and 7.62x54. I have far more invested in the ammo than I do in the rifles. I shoot regularly, and most of the few rifles I have are used regularly.
About the only reason for acquiring anything new (or used, though new to me) would be to complement what I already have in some way. For example, for a long time, the lone rifle in .308 was an Ishapore Enfield. A few months ago, I added a GSR, and with optics it is currently my favorite shooter. But I wanted a .308 semi-auto to complement the bolt actions, and that is what is on order: a CMP Garand in .308.
So after that, what else? Well, my .223 and .22 LR are both semi-automatic, so I might keep my eyes open for bolt actions to go along with them. But after that, I'm probably done.
Unless I decide I need another .308 semi-automatic, like a M1A1 Scout Squad or a VIPR. But really, there is light at the end of the tunnel. I just know there is.