botan9386: What does GOG gain by restricting such a thing to their launcher? The people who buy games through Galaxy and the people who buy games on GOG.com are using the same service. There's no incentive that I can personally think of to try and push Galaxy this way.
The incentive for GOG may well be survival.
If you use Galaxy to do the Download and Install, rather than download the Offline Installers, you remain reliant on GOG, and your web connection to them.
It would also be the case, that many game providers want you to use Galaxy, for its additional features, some of which they can take advantage of. Galaxy would be a selling point, when GOG try to make deals with game providers.
It is also a client like most are used to using with Steam etc. So it is an additional selling point for potential customers, and keeping many old ones.
So GOG want you to use Galaxy, especially now as they have built their business model around it. No doubt the majority of GOG customers use Galaxy, many of whom never bother to even backup (archive) their paid for games.
So continuing to support Offline Installers is an additional cost, and we know what happens with added costs over time.
GOG could well reach a point, where Offline Installers go the way of the Dodo.
It really boils down to how many customers they would lose, and the cost of doing so versus the cost of continuing to support them.