[rezonant]

I would be extremely surprised if Valve attempted to block Epic from doing that.

[smachiz]

Really? They specifically don't allow distribution of anything but games, which would preclude a storefront, presumably.

[int_19h]

Steam has an entire top-level category in the store for software.

https://store.steampowered.com/software

[rstat1]

They distribute games from both EA and Ubisoft that force you to use both companies launchers, that also just so happen to come with there own stores attached.