" Except you only pay once and "own" the license for all future builds, I guess."
That's the big difference. It would be like paying one time on Steam and never pay again for having the update "in advance". I don't think it would neither feel fair for the sub on SStar/Patreon, nor it would be fair if i put the game on steam with a big price to compensate.
I just don't see either of this happening, unless we do some sort of Deluxe edition that we never make free after the game is finished. And even then it would have to wait that the game is finished.