I'm one of those collectors who feels that 99.9% of the best and potentially re-watchable movies that will ever be made...have already been made. Maybe one or two new ones can be added to the list per year, but rarely more than that, imo. So adding one or two more per year to the collection, at most, doesn't create much of a problem. I'll still see newer movies in theaters, stream them on Netflix or Hulu+, try to catch up on the newer stuff to see if there is something there worth adding to a collection, Again, maybe one or two per year tops, as far as I'm concerned.
The benefit of feeling that way about it means you actually can own on DVD or Blu-ray today almost every title that you think falls within that "keeper" range. I also agree with others here that some titles are just fine on DVD and don't need to be upgraded to Blu-ray. I have about three dozen Blu-rays now and can only think of a dozen more movies I'd double dip on Blu-ray when they become available; some Hitchcocks, of course, Lawrence of Arabia, that kind of thing. But not everything.
So how manageable is a collection of, say, 2000-2500 DVDs/Blu-rays of movies made since the first decade or two of the 20th Century that you might like to keep within arm's length to watch on a whim at any given moment on a fairly decent 100" diagonal home theater set up? Pretty darn manageable actually. Particularly if you can part with the ordinary, unremarkable cases many of them come in and only hang onto a select few of the big deal special treatment cases.
I picked up 6 DVD/Blu-ray storage cases at Frys. The three ring binder style. 400 disc capability each. Slipped the bare discs in the sleeves more or less by genre, sometimes by star, whatever. They take up about as much room in a closet or out in the open in the home theater room as a small to medium size divan. Line 'em up on the floor against one wall and they'd never be in the way of anything. They're black so you don't even see them when the HT lights are dimmed. Want to watch something, you zip open the likely case and flip through the pages until it shows up. Sometimes you see something else along the way you'd forgotten you had and decide to watch that instead. No problem. I realize some day the latest-greatest media will either be the size of a postage stamp or every movie ever made will be available to download in pristine HD for a dime or stored in a key fob for $20. "some day".
But for now the collection of just about everything I'd ever imagine I might want to watch or re-watch again with others or alone is all contained in less than 6 three ring binder style storage cases (I've got unused pages of sleeves in each) that can easily fit in two medium sized 18.5"x18"x16" moving boxes you can get at Home Depot for less than $2 each. They'd be heavy but can be transported just fine that way, like books. I know because I've moved from one home to another and the entire 2400 or so DVD/Blu-ray collection (other than the handful of special treatment cases I mentioned) was carried over by hand in two arms full.