And every single one of those apps that developers have made for Windows 7 are not "Metro" and not eligible for the Microsoft App Store or to run on mobile devices. Only a few of them are even dotnet. The rest will require a near complete rewrite.

What he meant was: if Windows 8 sells like Windows 7, and it will, there will be a mind boggling number of users to target. Windows 8 market will quickly be bigger than Mac OS X market because Windows 8 will ship on every new PC being sold.

And since there are no apps, there are lots of opportunities just writing the same kind of apps that people have written in the past, except for Metro. It'll be a new gold rush and you can trust me, developers will rush for that new gold.

Well, I don't think anyone's expecting it to sell like Windows 7. They're expecting it to 'flop' like Vista. i.e. sell quite a lot but have a large user base sit on Windows 7 waiting for Windows 9 as people sat on XP after Vista.

Or, if applications were engineered properly, only UI part will need to be updated.

Have you ever tried porting code to Dotnet/CLR?

I think the point here is that if you have a properly architected .NET application, porting to any of the platforms supported either by microsoft or mono is relatively straight forward. So that would include Windows (all flavors), xbox, windows phone, os x, iOS, Android, linux, playstation, wii, and whatever else the mono folks are cooking up.

Are there any major publishers (or a large number of small ones) shipping apps that use Mono to run on non-Microsoft platforms?

Unity engine does it: http://en.wikipedia.org/wiki/Unity_%28game_engine%29