A BCP that recommends rewritng all prefixes to IGP will result in the following world:
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We should _not_ recommend _rewriting_ prefixes to IGP, but update the section of 4271 where _originating_ the prefix makes it incomplete. Thus all networks which would implement this BCP, would simply always originate their prefixes as IGP, and these who don't, would keep suffering of crazy misrouting. Later, like 5+ years in future, we may push a draft advocating for treat-nonIGP-origin-as-withdraw. Basically the same way as with AS Sets.
But! Maybe this method is more likely to get us to the point where most networks do have the code changes deployed, because at some point, either people do firmware upgrades, or they get new hardware (with newer firmware), with the code changes. Whereas optional BCPs, people are free to ignore forever.
Considering that the transits are _motivated_ to rewrite every origin to IGP anyway, I think that anything else would disappear from the world sooner than later.
We could write a draft BCP and throw that into an IETF WG to see what happens (to see what feedback / backlash / praise comes out of it). Who'd be interested in collaborating on such a draft?
*raises hand* Maria -- Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.