Maemo 6 will most likely be released for the N900 for various reasons. As already pointed out multi touch is a non issue, if you can't design an OS which doesn't fail as soon as a new input method arrives "you're doing something wrong".

If we entertain the thought that the N900 has been doing pretty well for being an N series device ( which judging by the activity on the boards and numerous rumors, it has ), then from a business pov it wouldn't make sense to leave the N900 behind unless the next device is very far of. It would be a disaster from a PR pov for a OS whose whole point is leveraging the open source community since their developer workforce by and large is also their market.

For me personally, I won't upgrade to a new phone in a very very long time for various reasons. The largest being that I believe the N900 will be the last device which has the hardware setup I want ( physical keyboard and resistive screen, multi touch is **** and I want the precision of the stylus ).