Assuming M6 ever becomes available for the N900, I can give my N900 to my brother while I trade up to the new M6 device (he tends to upgrade on a cycle about 12-18 months behind me and typically takes my old phone, currently an N85). With my N900, my brother can continue to buy more M6 applications from the Ovi Store as they become available, and I can do the same on my new M6 device.

However if the N900 can't run M6, and it won't be compatible with all the new apps in Ovi Store then while my brother may still want it, the selection of apps in Ovi Store that are available to him will steadily become more limited and in turn the income generated from him will begin to approach zero. Or my N900 may go into a drawer (I'm a hoarder, don't ask) if some of the more annoying bugs are never fixed in M5 as I won't want to be held responsible for giving him a sucky phone (he's not interested in being a Maemo beta tester), but either way Nokia will not see any more money from my N900, whereas it could continue to be a revenue generator if it could be given a new lease of life with M6.

The competition have recognised that while compatibility across devices may lead to reduced income from device sales (compared to the old "obsolete quickly, sell many" device mentality), this multi-device compatibility massively increases the size of the potential market for application sales AND it means that content suppliers can target many more end users, making the platform more popular which in turn leads to more device sales. Everyone is happy.

Literally every iPhone application works on every iPhone model - iPhone developers are not faced with having to choose if they want to support just the latest 3GS model ("and to hell with the other 20m+ non-3GS devices, didn't care about them anyway!") It's why the iPhone is so attractive to developers and content publishers alike - it's a stable, progressive platform and not one that chucks the baby out with the bath water with each OS release.

Leaving the N900 stuck on M5 with no M6 compatibility just makes it all the harder to convince developers and content providers that they should support the new M6 device and M6 OS, whereas making M6 available for the N900 means there will be a ready and willing client base (ie. market) from the word go.

Not making M6 available to N900 owners would be just sheer bl00dy madness.

Anyway, last comment from me on the subject for the time being.