is it possible to make a proof of space that is stored entirely in RAM?
It's technically possible, you just need to mount your RAM as storage partition.
in my opinion proof of space has failed because storing in hard drives is not practical and ends the useful life of the hard drive.
That's valid concern, but these days high-end/server SSD are more popular option among those who perform Proof of Space.
it would be fairer than proof of work.
Could you elaborate further?