A new industry in jail-breaking PC's is just around the corner probably.
Or a new generation of PCs where Google Chromium sneaks in and takes 50% of
the market virtually overnight. Sort of like they did with Android?
How often does someone come in and take 50% of an existing market? More
likely is that people invent a new market, duke it out for a while, and
eventually a winner (or clear leader, if you will) is born. Microsoft won
the desktop market. Google won the search market. Apple won the MP3 player
market. Amazon, the e-book reader market. We may still be in the 'duking
it out' phase for smartphones, though you could choose to look at it
high-end and mid-range (Android's gains are at the expense of the feature
phone, not the iPhone).
Windows 8 is interesting in that it is trying to use one thing on both
tablets and PCs. Windows 8 is obviously a response to the iPad - and a
response which considers UX and design far more than anything the Windows
group have done for years - but is there a tablet market to come? (Many have
said there is no tablet market, there is only an iPad market, and I have an
ignored TouchPad somewhere to prove it.)
Depending how all-in they go with Metro, the corporate market may have a few
years of staying with Windows 7 before moving to some sort of new thing.
Could it be a Linux on x86 hardware? Possibly. It could equally be Mac, or
web only on any device.
You can't run other OSes on the iPad, but that doesn't stop people buying
them. Companies will still make hardware with open firmware - though yes, it
might cost more, and old computers don't suddenly stop working. Or just go
buy a Raspberry Pi :)