I do not think the new Nxxx will be bad sale if N900 has Maemo 6 upgrade. Think of iPhone/iPhone 3Gs. iPhone 3Gs still sold very very good, even iPhone could have OS upgrade. That's because the new service or new software only works on new device.....That's the point.

but if N900 has Maemo 6 upgrade, N900 still could have another benefit. ex: full QT 4.6 base framework (??), full portrait mode, or new home screen ?

But it all depends on Nokia.