A lot of work was put in the past 5 years [1] to separate the consensus critical logic in Bitcoin Core. The result of this work is libbitcoinkernel, a library encapsulating Core’s validation engine. The primary purpose of this library was to untangle the consensus-critical part from the rest of the software, but it also makes it possible for others to build competing node implementations while still giving reasonable consensus compatibility guarantees to their users.
There is no need for a Bitcoin hard fork to allow for competing full node implementations. But short of something like libbitcoinkernel being available, users of these re-implementations hard forking themselves out of the network is indeed the most likely outcome.
However if you are hinting at reimplementing Core with a scope such as the one i am proposing in my post, i think this is a terrible idea. It would just further dilute the already scarce resources allocated to the maintenance of the network.
Work was put into it before then too but in 2020 it became its own sub-project and an explicit priority of the group. ↩︎