RFC 1918 describes space for entirely private networks, not “private extensions” to the public globally routable internet. There’s a subtle but important difference. The private addresses strictly aren’t routable on the public internet. NAT (which there’s also an RFC for) is what what allows packets to flow between private disjoint IP networks. IP is only scoped to work within a single network, whether it’s public or private or triangle.

That screws up when two private entities merge and discover that they're using the same RFC1918 block.

(or just want to interoperate, even)

End-to-end principle disagrees with you. As does my network. All routable IP space baby. Makes things a breeze.