fork was alright before other people tacked on multiples of cruft like threads and whatnot onto commercial unixes and they became mainstream. the current problem is that you don't want to have to copy all file descriptors if all you're going to do is call "exec" and reduce them to three: in, out, err.

for example, here's the caveats section from the macOS fork man page:

     There are limits to what you can do in the child process.  To be totally safe you should restrict your-
     self to only executing async-signal safe operations until such time as one of the exec functions is
     called.  All APIs, including global data symbols, in any framework or library should be assumed to be
     unsafe after a fork() unless explicitly documented to be safe or async-signal safe.  If you need to use
     these frameworks in the child process, you must exec.  In this situation it is reasonable to exec your-
     self.

That spells defeat :)

Earlier in the game, copy-on-write had to be created for the same reasons.