It makes no sense for OKC to trade CP3 to Miami, just like it didn’t make sense for OKC to trade Westbrook to Miami.
OKC holds Miami’s 2021 and 2023 unprotected draft picks. By making Miami better those draft picks will lose value. Even if CP3 is a bad contract eventually he will come off the books, but those draft picks won’t get better if Miami has a good team.