It's hard to believe it's impossible. Maybe Core should start breaking the main repo down into libraries to allow some interface specification of the consensus part? Sure, It will need additional financial support to break it down, but once done,  making things compatible can be assured simply by comprehensive coverage of test cases. And once the main interface is covered, new implementations can start to flourish.  

 There’s ongoing work to do that! Look into the libbitcoinkernel project. 

 Nice! Thank you!