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?