But in terms of acting like a good Mac app, it does not.
Not just Photos. Apple also replaced iWork on the Mac by a codebase that is shared with the iOS version. Years later, it is still missing many features and a poor imitation of its former self. I still regularly look for I feature I was sure that was there, but hasn’t survived the iOSification of iWork. For example, Pages was a great poor man’s DTP program. Unfortunately, the ‘upgrade’ removed linked text boxes and it took them four years to bring back this functionality.
If Marzipan is really realized, I think macOS apps will definitely become an afterthought, the iOS market is many times larger and its economically simply too attractive dump an iOS copy on MacOS. Electron apps have shown that many companies go for the lowest common denominator for economical benefit.