I'm not really sure what you are arguing for or against.

No one's against having Maemo-based mobile phones/internet computers in the market. In fact the more the merrier! (wish there were even non-Nokia models with Maemo come to think of that)

Would it invoncenience you, as a user of the Nokia N900 integrated phone/tablet, if there were other, cheaper models that would simply tether (or use WIFI) to get connected, and which were occasionally used as plain tablet computers or media devices or whatnot?

I don't think so.

What if the availability of the non-phone "phone companion" tablet model(s) helped make Maemo a more popular platform, resulting in more software also for the Maemo phone users? Would that be troubling?

But if you're happy with Nokia's exclusive market segmentation of Maemo, well, good luck with everything. Although I'm personally not interested in the current exclusive Nokia/Maemo offering, as an environmentally conscientious person I hope that Nokia and the Maemo ecosystem sustains your device beyond the next year or two. If the Maemo community was larger maybe the support would last longer yet and the software selection might be greater too.