Qt is an application framework which can be used as an abstraction layer between code and an OS sort of like Java so that you can write software for many systems at once. It's been around for quite a while and its most common use was writing software for Unixy systems and Windows at the same time. Its association with maemo and meego is a very small part of what it is.

As for nothing running except Qt on the N900, that's simply untrue. QtIrreco is a good example of software based on Qt running on the N900. Unless the next meego device has an IR transmitter, it's only going to be able to run on the N900, too, even though it could be "ported" pretty much by recompiling it and repackaging it for future devices (assuming LIRC supports them, too).

Saying Maemo 5 is a dead end is like saying Windows XP is a dead end. Vista and 7 have the same app frameworks that XP had so when someone goes to write software, unless they need to use something specific to Vista or 7, they are, by default, writing software that works on XP as well. Sure the newest games don't run but it's not like your current software breaks. I imagine your N900 will break long before the protocols that the software is based on changes.

As for meego running on a TV... Android runs on the nook but you don't see it and you can't go to the android market. I imagine meego running on a TV will be meego with a heavily modified interface running on a TV and it really won't make any difference to the N900 since you won't be watching TV on your N900 or controlling it with a remote.