It /was/ a drop in replacement for a single point in time but it isn't if you make use of any of the recent improvements openssl has added. For example auto selection of DH/ECDH primes and curves.

Note that recently a big clean up of the openssl codebase has taken place so openssl master no longer exposes the internals of structs etc. meaning it's both more auditable and more maintainable. This code is not yet released however.

Well that's the intention... is it really in practice? They have deliberately removed a lot of support for rare architectures and features.

Distributions such as CentOS/RHEL which are focused on stability are not going to replace OpenSSL in any existing releases.