You can do that currently. Steam already supports the exact process you described: top up your steam wallet and buy games with steam wallet balance. Actually, there are things you can only buy this way (some in-game items, not sure if it's to workaround gambling accusation or just coded so for no reason).

The issue is Visa/Mastercard/whoever is pressuring Valve isn't happy about the very existence of incest games. They don't want to be associated with incest/rape even indirectly.