What prevents that from catching on at scale is, the "big boys", like MS, FB or Google, mostly not playing ball and never implementing these in their own messaging platforms, to keep their gardens neatly walled from each other.

As intraplatform exchange is not really in-line with what most of these platforms are striving for these days; Interactions with their own platforms and the advertisers on it.

I have yet to receive spam on my Mastodon or XMPP address (which I treat like my telephone number).