The downside of this proposal is that it is tightly coupled with class creation process.  It requires all existing class decorators, metaclasses, and code generators to start caring about something that isn't part of their core functionality (very few tools are ABC aware).

I'm usually dubious about proposed solutions to problems that 1) rarely occur in practice and 2) require everyone else to change their code.

Am not sure why total_ordering was mentioned?  No current collections ABC requires the ordering methods and hand-rolled numeric classes typically don't use total_ordering because it is simpler, faster, and cleaner to implement the methods directly.