Do you do any alerting for INVALID indexes? For example, by default PgHero will display them prominently and I believe PgAnalyze does as well. My thought is to put the energy into making INVALID indexes highly visible. Perhaps combined with a process step. 1. Any CREATE INDEX CONCURRENTLY migrations go out in their own deployment. 2. Any queries that depends on that index being present means a PR has a process step asking the author to verify that the index exists and is valid. That way you wouldn't have to lose if_not_exists.

That all said, still would be cool to make this the default in Active Record. Nice idea!