I agree up to a point - demand creates the supply - but the manufacturers bear their share of responsibility too.

Sermon for the choir: If they don't choose their components carefully, or develop open source (or better, in-kernel) drivers for them, the devices end up very restricted, and are effectively useless once the manufacturer stops supporting them.

I've read about Dell releasing 'Ubuntu certified' laptops which are tied to a particular kernel version, and hence a particular release of Ubuntu (at least unless you're prepared to undergo a lot of faff trying to forward-port old kernels). I'm not a big fan of Ubuntu, but I wouldn't say having to use it is fatal. But not being able to upgrade to new releases I would definitely object to - I'm still using a Thinkpad X201, and intend to keep using it till it's actually useless or broken (funnily enough, I booted its original disk a couple of months back, to upgrade Windows. It claimed the battery was still at 99% health or something. Probably more sensor error than the fact that I've almost never unplugged it, but at least a bit of both... )

Less of a concern with an ultra-mobile, which you expect to have a fairly limited lifespan anyway - but still a concern.

eta: of course I suppose in a way tied in to your point about people needing to support the manufacturers, that's a self-defeating cycle: if a manufacturer makes something that you can keep upgrading, they don't sell as many as if they can force obsolescence. The solution isn't obvious - maybe an electronic waste tax?