To address multiple points:
Resources are finite. That means all the energy, funds, et cetera given to Galaxy is unable to be used on alternate strategies. In other words, the more resources that go to Galaxy, the less resources available for strategies other than Galaxy. The same is true of singleplayer games versus multiplayer games, or even different modes within multiplayer games. If the funding is a sunk cost, it would still be better to focus on other strategies rather than doubling down. To double down would be to commit the sunk cost fallacy.
A point I've made many times: A mom-and-pop store cannot realistically compete with Walmart. If trying to adopt the same features as Walmart, the mom-and-pop store goes out of business as it cannot price the same as Walmart nor does it have the market recognition (even if it started to gain a foothold, Walmart could still likely undercut it). Where the mom-and-pop store can succeed is by catering to a niche better than Walmart can, or perhaps by providing better service in a more subjective sense (e.g., more eco-friendly, more appealing to certain politics, et cetera).
More importantly,
Just because GOG's "old" audience was possibly deemed tapped out, does not mean that the solution is to become Steam-lite.
A couple subpoints to this, which tie into one another:
One could point out that general consumers are also "tapped out" on Steam and Steam-like features. Look already at the outrage caused by Epic, or by having to use other proprietary clients. And for all the businessminded "businesses are about making MONEY, not satisfying you", are you aware of the term red ocean? Because even if consumers were loving every client that got released, GOG would be entering a red ocean market by requiring their own additional proprietary client. As it is, GOG is dipping its toe in the red ocean, not drastically harming business, but not necessarily helping long term either.
Right now, there is an absolute glut of DRMed games. There has been a buildup to numerous clients and services, which is poised to turn into a glut given the "access over ownership" cancerous philosophy that publishers are doing handsprings to implement as much as possible ASAP. The overarching point here is that it is extremely difficult for a business to enter a red ocean market and succeed, primarily because they are not standing out. Hence the need for a unique selling point. And no, "mandatory client, except our mandatory client interacts with your other mandatory clients", is not enough of a USP even to the braindead general public.
More importantly, the glut of DRM and clients has one possible positive in that it is opening the door for a new audience of DRM-free gamers. One has to wager that eventually, a significant number of people will get fed up with this stuff. It would still take some large scale outcry, but is certainly possible. In the console realm, people have switched to and from Nintendo, switched away from Playstation after PS3 reveal, switched from Xbox brand due to the infamous "red ring" error, switched from Sony after PSN hacks, switched far away from original Xbox One reveal, et cetera. Companies, especially ones hasty to greed, are not infallible.
Already, people are becoming fed up with how video streaming has turned into multiple competing clients and brands. If that were to happen where suddenly there are numerous PC competitors, or even the exclusives were redistributed among the existing stores, there is a chance that these folks will just wake up to the fact that mandatory clients are anti-consumer garbage. The way GOG is introducing Galaxy 2.0 strikes me as a cautious step in recognizing a future of many competing clients; GOG is able to interact with all of them. Unfortunately, this does nothing for DRM-free gamers and would be more a move to survive, rather than thrive.
I still think GOG, as a bastion of DRM-free and the only place to buy large-scale DRM-free games, can thrive.
