I think it "might scale back to sensible levels". ( When you look at the 1990s for example, how many people had PCs back then.)
In the private home: Perhaps 10-30% of the population?
Geeks and professionals will still need PCs. People who only use the Internet, browse, shop and watch movies will probably no longer need a PC.
In the workplace:
Before the PC most "screen and keyboard" type workstations where dumb terminals. Then there was the PC-wave, but now they have gone back to somewhat-dumb terminals. In our company we used to have ~300PCs 10 Years ago, now we have about 20 PCs and 1000 Terminals. (Which of course could still be classified as small PCs without much storage running Linux, but you can't really install much on them besides client programs to connect to servers)