Probably just want to be able to control the user experience honestly. It's sort of what your average "consumer" expects. Maybe they could fork Heroic, but I don't really think that benefits either party long term, but I'm not super intimate on the long term life cycles of open source projects, perhaps there would be enough overlap still to offer good opportunities for up streaming despite the difference in design direction.

They want to spend more money on their own launcher instead of just profiting from the existing FOSS one?
Couldn't they just buy some irrevocable licenses of ancient games to actually enable eternal preservation instead?

Just an extra piece of software of no real benefit to me. I'm only interested in single-player offline games, and I prefer to organize what I have using a directory structure rather than someone else's GUI. I try to avoid installing anything I don't have any use for to avoid unnecessary disk bloat and just in case it drags along some daemon with a nasty security bug. If you actually want the features Galaxy offers, I'm not aware of anything evil about it.

Not the previous poster, but personally I prefer that the application I use to organize my games collection and launch games is store independent, so something like Lutris or Heroic Launcher.

The independent launchers actually integrate with multiple stores, don't constantly shove adverts under your nose, aren't designed to try and make you buy more stuff or be tied as much as possible to a store and, at least in my experience of using Lutris and Steam side by side, tend to be way more flexible in terms of configuration around and use of your games (for example, I can launch my games in Lutris sandboxed with networking disabled using with something like Firejail, which I can't with Steam).

Frankly, even only the first of those things - integrating with multiple stores, thus supporting my entire games collection - would be enough for me to favor independent games launchers.