The trade-offs here are incredibly complex (and are mainly a matter of deciding whose code and configurations we want to break in 3.7+), so I think competing PEPs are going to be better than attempting to create a combined PEP that tries to cover all the options.

That way each PEP can argue as strongly as it can for the respective authors preferred approach to tackling the default C locale problem, even if they point to a common background section in one of the PEPs (similar to the way PEPs 522 and 524 shared a common problem definition, even though they proposed different ways of handling it).