I'm not sure anybody has ever felt memory allocation was a solved problem. But more that it's a difficult one to do well for everybody.

You can build fancy balancing trees, heaps and all sorts to try and counteract fragmentation, but then you risk messing up cache coherency and you may spend a lot of time doing tedious busywork that can hold up real work.

What we have now is subobtimal for sure. Particularly if you come along and allocate huge swathes of memory in userland and then release it, only to re-alloc it. Huge page tables help a lot here, from the user/kernel side of things. But of course that is also not a perfect solution. Most apps don't want huge page tables as they don't map well onto normal things like browsing and listening to music. And so it goes; round and round.