I could've changed that to almost any OS or software. It's just the way things are done. Even in open (or in this case, I suppose you could say semi-open) development, as qgil plainly explained, sometimes desired changes go too deep and are too difficult to implement without just saying 'Screw it, we'll hafta go for it in the next version.' All - hey, not even most - bugs and feature requests are ever fixed or added in existing versions of software, usually just what has to be done.
That's not quite right. Let's replace my comments with Ubuntu Intrepid and Ubuntu Karmic koala. Sure, intrepid gets orphaned - but I can then use Karmic. Mostly, this same can be said for windows.. but with windows you pay for an upgrade. However, windows offers discounted upgrade versions of the OS for previous customers. So for your argument to be taken seriously... Nokia would need to offer an upgrade device for cheaper to those that already have an N900... or, of course, bring M6 to the N900 and give us the option of loading it.
Relax and enjoy your N900s. Upgraded or not, either way I'm a happy camper.