But the reason this happens is that most people, even when the computer they owned really was a general-purpose computer, didn't use it as one. Anyone reading or commenting on HN is an outlier; we actually need a general-purpose computer, because we need to be able to do things with it that its designers and builders never thought of. But most people don't. Most people want a small and pretty much stable selection of apps, and if those apps are there, they don't care whether or not the device they're running on could also run an infinite number of other apps.
As an anecdote: we have an iPhone 3G in our household, a phone that became progressively worse with certain updates after the release of both the 3GS and 4, to the point that it is nearly unusable for surfing now. This leaves me with exactly two options: stop using it as it was meant to be used, or buy a new phone. And for the other camp: in order to get an inofficial version of Android that is newer than Gingerbread on my phone, I would have to reformat its internal storage, which I can only do since it is unlocked by now. However, even that is not entirely without risk, and it is not supported.
Oh, I entirely agree; I've been bitten by this myself (and on desktops/laptops, not on mobile devices), and it's one of the main reasons I don't have an iDevice and don't use many apps on my Android tablet.
I'm just saying that you and I are outliers; most users appear not to care that their device is locked down and they will sooner or later be forced to upgrade. (Many of them are used to upgrading every time their phone carrier comes out with a shiny new toy anyway.)
As long as i can still buy a general-purpose computer for $200 that can do/run anything I want, or pay for processor cycles like a utility like I can with AWS, this will seem like computing purist's paranoia. It's a lot like Stallman's regular speechifying, when it strikes me that while he's not completely wrong, he's not really right either.
The use of the word pretty contradicts your assertion that they are not locked down.
You cannot simply buy the RPi at a normal retailer, unlike (smart)phones, laptops, (smart)TVs etc. etc. This does not mean the RPi isn't a fantastic little machine or that it doesn't offer new possibilities. However, there is a world of difference between a developer board (that doesn't have a case, operating system, built-in storage etc.) and a ready-made consumer article.
General purpose computers become rarer with an increase in phones and tablets, all of them locked down. This is the making of a niché, and I think it is dishonest to simply assume people are worried about losing their status. Is the reduction in sales of general purpose computers the end of the world? Perhaps not. Should it prompt closer scrutiny and some concern? I think it should.