I think it's a great idea to deprecate crypt(). I wouldn't want anyone to use it in a new code. For legacy applications, we are giving them enough time to upgrade their password storing policy. Also, it's not like we are removing support for hashing in general from PHP, we are just deprecating a function that should not be used for password hashing. There's a suitable replacement and it's also possible to create a shim for the crypt() function if one really needs it.
I also recommend reading this article https://www.michalspacek.com/upgrading-existing-password-hashes