Mind you, a lot of sellers in Europe do actually support paying by bank transfer (which goes via SEPA) but a lot don’t

This is not about what “a lot of sellers” do, it’s what Valve could do as an easy alternative in one big region of the world.

though if you do the bank transfer from a banking app in your smartphone it’s reasonably simple plus some of those payment systems are really just a convenience layer - say an app scanning a QR-code for automated payment - over the whole “open the transfer screen and manually enter 20-something digits and an euro amount”

Making SEPA money transfer by scanning a Qr code from the bank’s app is literally a thing. That’s how I paid the dentist a couple of months ago.