I am not aware of a "multitude of specs." I know of three. 1. NIP-04, which should only be used with auth relays, due to the metadata it leaks about both sender and receiver. 2. NIP-17, which definitely has the shortcoming of being unable to filter DMs based on who the sender is, since the metadata with sender info is hidden. More private, but yes, more susceptible to spam without some other mitigating factor. 3. NIP-EE, which uses MLS and may well be the future of DMs and group messages, but is not yet ready for prime-time. There are a couple clients that are doing a bit of their own thing, too, like @Keychat, but that's outside of any NIPs that I am aware of. As far as whether I find any problems with your argument, only a couple. First, you already identified why it will never happen. Users expect to be able to DM for free. Requiring payment to send a DM is a fantastic way to reduce spam, but only if users are on-board with it. I also don't think it would result in a convergence on a particular spec. If anything, I think it would result in more fighting about it. You would have folks like us on one side, shouting about how requiring paid "postage" is the most effective way to reduce spam, especially if users themselves can set the amount of postage required to DM them, while others would be shouting about how making people pay for things is a barrier to adoption, and stubbornly keeping DMs in their clients free for everyone. Heck, Keychat is already an example of a client that has MLS-based DMs with paid postage, yet its existence hasn't resulted in every other client converging on that spec, because most users simply don't want to pay to send a DM, even if it would mean they receive less spam.