From the post:
"That’s a pity really, because even though [Paul Graham] says eventually Microsoft may encounter problems (who would have thunk?), [he] effectively concedes that Microsoft is going to make a lot more money in future. And some people are going to get rich competing with, and taking a chunk out of Microsoft’s revenues - although apparently not Graham’s start-ups because it’s unfashionable to even try."
Microsoft's flagship product is the Windows OS, but supposedly they don't profit from its sales [Edit: this is incorrect, it's actually Windows and Office that keep MS from major losses]. They keep developing new versions of Windows in order to maintain their share of the market - those customers will then pay exorbitant prices for other products (e.g., MS Office) which sustain the company.
The problem is that the OS market has been commoditized and Microsoft Windows is an inferior product. Sun open sourcing Solaris is an indicator of OS commoditization and is Windows OS objectively better than other OSes? Assuming Microsoft continues down the path of producing OSes that barely meet status quo, it's only a matter of time before they lose their grip on the desktop market. Of course, maybe they'll surprise us all and build an awesome Minix-based OS.
If MS loses the desktop wars, they will end up where they started, writing third-party applications for Apple. And there's always web application development.
It's not only that web applications are sexy, it's that the OS wars require a large amount of resources (people and/or money) to win. How many Windows users choose Wordperfect over Word? How many Apple users choose Powerpoint over Keynote? It seems pretty futile for a third-party developer, especially a startup, to attempt to beat first-party applications.