Wow, that comment hasn't aged well! I wasn't here at the time but it's almost like those of us who don't need a client and cautioned against it were right. And we'd have much rather been wrong.

TLDR most replies.

I have no illusions GOG will listen to the concerns some us have, if they hear them at all.
As long as they keep selling DRM free games on GOG.com, give access to offline installers and keep Galaxy optional for both of that I'm happy.

Whether we like it or not, with the growing numbers of more and more powerful phones, tablets, etc. the demand for apps increases. I see Galaxy as kind of a predecessor of an app.
Personally I believe that at some point it will be inevitable for games stores and publishers to offer their launcher/store as an app for PC, Android/ and iOS/MacOS in one form or another. Though most will probably at the very least await the law suit between Epic and Apple about the 30% commission.
I also will not be surprised if GOG.com at some point starts selling mobile games. Curated of course.

I can only assume they'll make very clear you're buying games on Epic's site and require an Epic account for it. If not, I'm sure sooner or later (most likely sooner) there will be quite some backlash about people thinking they bought a game at GOG.com. Not to mention Epic also won't want any confusion about it.
GOG.com is probably hoping to close similar deals with other stores as well, hoping that in the long run it will make them THE starting point for buying games. It might be a way to get more titles on GOG.com in the long run, which might end up in a win for all of us. No matter what happens, I won't lose my games. Nor will I lose access to them.
All games I purchased here are stored on an external drive.

How? Developers and publishers are surely aware of the GOG.com website right now, in the present day. In most cases they couldn't care less about providing a DRM-free release here; instead, they just continue to sell DRMed games at other places. How does being able to purchase the DRMed games from Galaxy 2.0 store result in getting DRM-free releases on GOG.com in the long run?

If their sales volume grows, even though through sales from other platforms, they'll eventually become more interesting for developers and publishers. Not just as an additional store front, but if volumes are large enough also for selling on GOG.com.
I have no idea what % they're getting as a reseller, but we can all be sure it's won't be too much. Downloads etc. still go through the store the title is on.
Ubisoft, EA, etc. can however easily close a similar deal with GOG as Epic did. They'll all rather pay a small resale % to GOG than giving 30% to Steam. To them any sale that's not going through Steam directly would be additional profit.

It's unlikely we'll get more AAA titles anytime soon if ever, but the chance we'll get more of them after a few years increases as GOG.com becomes more interesting to developers and publishers..

I apologize if I am misunderstanding your point but I still don't follow. Remember, the narrative now is that GOG the company has both "GOG.com" (this website with DRM-free offline installers) and "GOG Galaxy 2.0" (the client with the new store where DRMed games will also be sold).

GOG the company's promise is apparently something to the effect that the games on GOG.com website will remain DRM-free whereas the only place GOG the company will have DRMed games is in this special wing of the client, which can even be hidden if users care not to browse those games.

How does higher volumes of sales of DRMed games via GOG Galaxy 2.0 get developers/publishers interested in making a DRM-free version for the (supposedly) smaller DRM-free GOG.com audience? Wouldn't developers/publishers, at "best", just put more of their DRMed games on the client store?

Honestly, I don't even think you CAN see this unless you have an Epic account, so the question becomes, who is this for? I doubt third parties will even consider what's on Galaxy because of this simple fact. After all, if you have an Epic account, you have the Epic client, most likely.

Honestly, I expect GOG's ability to get AAA games (for whatever that's worth) to remain unchanged, if only because there aren't going to be any significant number of people using Galaxy for non-Gog titles.