Another thing that doesn't get brought up that much is that, with regards to selling old games via digital distribution, GOG was something of a test case. It's probably safe to say that that aspect of their business model was a "victim" of its own success: it showed bigger storefronts and publishers that there was a significant appetite for old -- even obscure -- games, and so those began to be sold more and more on other stores. There are now game publishers whose business model is mostly or entirely centered on resurrecting old games in some form -- from "merely" getting the rights in order and re-releasing them in a more-or-less untouched state, to doing like Night Dive and also working with fan communities to make new enhanced editions and new PC ports of unique legacy-console versions. (There's also definitely a trend of making brand-new games with old IP, but that's less relevant to the discussion at hand.) Such companies understandably don't want to limit their sales by only selling in a niche "underdog" store -- they want to sell someplace like Steam, where all the customers already are. Thus, lots of people will just get their old games at the same place where they already get most of their new ones, and where their gaming friends already hang out. And GOG loses out.
GOG used to have a lot more exclusives than they currently do; these were all old games (naturally), many of which they did the technical work on to get them running on modern systems, and/or they did the legal-rights legwork to allow the games to be sold once more. For games where the publisher is doing most of that (and I'd guess that's the case with a majority of the old games released here these days -- assuming a fairly expansive definition of "old"), GOG doesn't have much bargaining power to get any sort of exclusivity, and that means, at best, a simultaneous release on both stores, and at worst, the game won't be released here for years, if ever.
If most bigger "general purpose" stores have some old games available, with one or two of the biggest of those stores starting to have lots available, and many of these games are the same ones sold by a store selling nothing but old games, then what's that store to do? If you're a niche game store whose main niche becomes more mainstream -- and consequently gets "invaded" by mainstream game stores -- the answer is, "Adapt, or die." (And if you fail to even try to adapt to competition, you will almost certainly die, be it quickly or slowly.)
New and recent games are cheaper to carry, business-wise, since the development studio and the original publisher (which, in the case of indies, is often the same entity) probably still exist, so you won't have to work out rights issues, and they should be the ones to handle compatibility issues, fix bugs, et cetera (emphasis on "should"...). Older games tend to require a decent outlay of resources up front, often with little guarantee on return. Little indie games are presumably less risky to carry for several reasons, and are less of a stone around the store's neck if they turn out to not sell well.
Well, this turned out to be quite a ramble. I guess the TL;DR version would be that GOG management didn't just randomly decide "We don't care about old games now LOL". No store exists in a vacuum, so competition must be taken into account; and there are different levels of expense for selling and supporting 20- to 35-year-old games and, say, 0- to 5-year-old games.
Post edited April 20, 2020 by HunchBluntley