Supposing what you say is true about spammers preferring rarity tied to higher expenses, even offering them a cleaner and cheaper way to spam will not convince them to forfeit the luxurious way of doing it, that is, with fake pubkeys.
So if that premise is valid, it contradicts your conclusion.
And if it is not valid, it does validate my point of only leaving the most expensive loopholes accessible.
No, people are not angels, people are a gross sum of praxeological individuals that work by profits and incentives.
More options to send spam inevitably means more spam.