There's plenty of R&D to be done on operating systems and systems software in general, but if you define an OS as merely a hardware abstraction then indeed ther

There's plenty of R&D to be done on operating systems and systems software in general, but if you define an OS as merely a hardware abstraction then indeed there isn't because the hardware is, almost by definition, hard for researchers to change. Nor is there much reason to do so. The problems that are interesting to research in systems are all at a level much higher than the kernel. They're around software distribution, distributed computing, user interfaces and so on. There's plenty of scope to do interesting stuff there but academia has mostly given up on doing this stuff, I think because better designs at higher levels aren't considered "research" by granting agencies/journals. Most of the interesting OS research is getting done by cloud vendors, Facebook, Apple and startups these days.