I can't figure out if I'm just too old and grumpy or if operating systems are just desperately uninspired.
It's probably a mix of both.
Of course operating systems have matured. Today they do practically everything we can think about. There are no obvious features left to add. So development, especially from an end user's perspective, seems slow.
On the other hand, I don't agree that there is no development like you seem to imply. I'm using OS X, so that's the only OS I can really talk about. Some of the things we got the last few years:
A fast global search can really change some workflows. Gone are the days when I had to trawl through nested folders to find that file from a week ago. Now I can search for name or content or even the date I did use it last.
- Time Machine
Switching machines? Just restore from the last Time Machine backup and everything is like it was before.
That new version of application X sucks? Accidentally clobbered some file? No worries. Restore from Time Machine backup.
- iCloud Sync
OK, so not everyone wants that. But it is nice if data is kept in sync between devices automatically.
Of course there is much more, many of it not directly visible. (There's a reason MacBooks have great battery live. And it's not just better hardware.)
So, I agree with you up to a point. OS development is not as exciting as it used to be. But it didn't stop either. Interesting things still happen.