> nothing will happen, because matrix uses strict versioning of protocol
Bingo! You've precisely stated the reason, why Matrix is a product and will always be a product, and never a protocol. In real world, third party servers will never be on the same version of protocol.
A few years ago [1] people here explained to Matrix developers that "single monolithic versioned protocol" that is NOT the way to build federation, ant that it is silly to assume that everyone will magically update once the new version comes out, but they were preaching to deaf ears.