groze: As for the "if we let the little things slide, then..." argument, well... let's just say it's not really something I like to concern myself with
for reasons.
Nice website, I think I'll bookmark that one, for my own edification :D
While I need to point out that games not getting updated properly or having content like multiplayer cut out from their GOG releases is by no means an "extreme hypothetical" (I'm sure you've seen the lists), you are certainly right that we have no hard proof which clearly demonstrates that the one kind of behaviour (not implementing achievements) directly leads to the other kind of behaviour (i.e. not providing updates). The argument assumes that people/corporations MIGHT (see my original wording) not improve their bad behaviour and MIGHT exhibit even worse behaviour unless you push back on it, based solely on personal or historical experience with people/corporations. So you're right, the argument is indeed logically unsound.
Regarding the point of calling a game with missing achievements "incomplete": I agree that some people can be extremely aggressive, pushy and even downright offensive about stuff like this, but I'd argue that it's still justified to call a game released without achievements on some PC storefronts "incomplete" if the same game is released on other PC storefronts with achievements. The game might be incomplete in a way that doesn't bother you or me personally, but it's still lacking features that other storefronts offer, that are important to some people and that could be easily added since GOG Galaxy does offer achievement functionality.
To give you another example: The GOG release of
Chains of Satinav only includes English text and voice acting, while both the Steam version and the original boxed copy also feature German text and voice acting. Since the game was made by a German company and is based on a German pen & paper RPG, German native speakers like me might prefer to play it with the original German script. So we would consider the GOG release "incomplete" while people who don't speak German might argue "this doesn't impact gameplay, so it's not incomplete", since they don't want to play the game in German in the first place.