Except that the 'good parts' of LibreSSL consist largely of two things:
1. Removing a ton of broken, obsolete, or poorly-designed code
2. Fixing bugs and incorporating fixes from the official OpenSSL bug tracker
In short, the 'good parts' of LibreSSL are a team of people willing to devote significant time and effort to making the OpenSSL codebase secure, clean, portable, and trustworthy. That's something that the OpenSSL devs either didn't have the time or energy to do.
As for interoperability: OpenBSD's process has always been to write a single core with a single target (OpenBSD) and then to have a separate team take care of platform-specific issues. This allows the team doing security work on LibreSSL/OpenSSH to focus on the results by writing a single implementation, and the porting team can spend their time porting that implementation to other platforms (rather than having someone implement functionality while trying to be aware of how each platform handles things).
It's worked well for OpenSSH so far, so I don't see a problem with using the same pattern for LibreSSL.