The marginal extra disk spaces used by flatpak really isn’t a concern for most users, much less valve. If you do everything in flatpak and your apps only use current runtime versions, the additional space used by flatpak is in the megabytes, since libraries like libc are going to be on your host no matter what.
Depends on if you want your data format to be strict ascii. If you don’t care, then sure, why not?