Back to the OP
I think there is room for all wheel sizes, but will the manufactures see it this way.
I had a couple of decent 26 bikes so I resisted the 29 for a long time, after an accident, I could see it would be a while before I would be back off road so I sold both MTBs, when I felt confident enough to get back off road 27.5 seemed to be the way to go but after a bit of research it seems XC riders prefer 29 as most of my riding is XC I went with 29.
The 29 is not as good for technical bits as the 26 wheel bikes were, but I am riding over things without a second thought, that I would have considered as obstacles before, it doesn't accelerate like the 26 but once up to speed its a flyer.
If I had been smaller I would have have considered 27.5 or 26, purely because I don't like the look of small frames and big wheels, but the performance may outweigh the aesthetics so who knows.