jamyskis: ... Not really, no, ...
I guess it depends on the point of view. Yes, it's not exactly the same but I don't care much about the differences because I think they are not so significant. I think if DRM doesn't make sense for single player it also doesn't make much more sense for multi player (although it is a bit different) and again, if it makes sense for multi player it also makes sense (maybe a bit less) for single player.
I think you described the differences as larger than they really are (and they are becoming even smaller) and you overestimate the costs of keeping the servers running. It's just a guess but I guess the one-time development costs of the games are still much, much higher than the ongoing server maintenance costs, so we do not really need to worry much about possible losses there. But I may be mistaken and would like to get more first hand information there.
For example, if you think you will want to play single player games 20 years from now then you should also consider that you might want to play multi player games 20 years from now. After all they are all games. But this will be very hard if there is no server to communicate with. On the other hand you could for example publish the server (after some time maybe) so fans can run their own multi player servers, then there would be no problem.
As I said, as a publisher I would just kill single player games and make them all online at least partly and if many publishers would follow what choice would customers have than to buy the DRM and play for only a restricted time.
Yes, that would be locking single player content behind a DRM walll, but the same way you can (and that is done already) lock multi player content behind a DRM wall by not releasing the server part, so you could as well justify trying to lock single player content too. I think that differentiating too much here between single and multi player does more harm than good regarding the impact of DRM and that's why I try to avoid it, but I guess one can see it differently. It all depends on the point of view. Obviously GOG chose the easy way out but at least they should label these games clearly as DRM.
Online authentification always was regarded as DRM, otherwise what meaning has the term "digital rights management" anymore? It's only DRM if we can advertise that we don't have it? This is kind of silly.