I wonder what the problem is, besides Nokia being content with the state of the OS as it is now.

PR 1.3 is a mature OS (as mature as Nokia is able to do). The hildon desktop stuff and some other things are open source and the community can and will continue to improve it now that Nokia stopped messing with it (they already stopped that in August).
The phone works as advertised and the N900 is a good phone for everyday's use.
Nokia's continued patch support for the N900 wouldn't change a thing with the Ovi store situation.
There are lots of applications available in the maemo-extras repository, and this is not going to stop soon.
The N900 is equipped with Qt 4.7 (which not even Symbian has right now). The only two consumer devices with Qt 4.7 today are the N900 and the WeTab (since 2 days ago) AFAIK. When doing cross-device development for MeeGo, developers can treat the N900 as MeeGo device. It works. But since this is still all young, it takes its time to take off.
Instinctiv need to make money for a living. The Ovi store cannot deliver that on the N900 and will never because there are not enough units sold for a critical mass. This is Nokia's fault, but it was also their intention to make the device available mostly to early adopters and developers, instead of going mainstream immediately. Maemo6, now rebranded MeeGo, was intended for Nokia's mainstream Linux debut.
They communicated that clearly before sales started and always tried to avoid making a hype of the N900.

Phone contracts are usually for two years but people who wish to change can put the SIM card into a new phone. My SIM went from a G1 into the N900 without problems.
Still, there's no need to abandon the N900 if Nokia decided to stop publishing updates.

And if you're missing 100.000 of apps (for what? trying one after the other in boredom because they're all crap? yes, then you'd need a huge number to cover a two year's time), the N900 has never been the right phone for you and would never become.