The thing I've never understood about this is I have that freedom with an Apple device. If I find that I'm being constrained in ways I don't like, I simply sell it and buy something else.

Absolute worst case (which is massively unlikely) I'm a few hundred dollars down (which I can afford). Realistically any unpleasant constraints would come in gradually and I'd just buy a non-Apple phone at the point I would have upgraded my iPhone and I'm not even financially down.

How am I constrained in any real, meaningful sense?

Well, for starters, if money is no object then everybody is free. It kind of defeats the whole point of the argument to take such an important constraint away and then pose the question "how am I constrained?". Keep in mind that Apple's whole ecosystem is closed, not just their devices. I can't read their iBooks on my Android tablet, play their games on anything but Apple hardware, etc. Even the connector is patented to control every single digital bit that travels to and from the device. So in 10 years from now you might be up for much more than "hundreds" but thousands or tens of thousands if you have accumulated your lifetime library of content there.

But more seriously, your solution works only as long as there is "something else". So you're predicating the whole thing on the fact that there's an external entity out there for you to flee to. The problem is that we're moving headlong into a dystopian future where there simply isn't any ecosystem left where you can go. The least worst option at the moment is Android because it allows side loading and is open source so if Google turns evil enough you can be sure that someone else will come in and fork it to compete with them (you can argue that already happened). You can even create your own builds and deploy them to your phone if necessary. Now I'm not advocating any such extreme tin-foil type things in reality, but I do think it's vitally important we preserve at least the ability or freedom for these options to occur, because they keep all the other players honest.

Suppose you've bough an iMac because iTunes works much better on iOS. Now you're down a few thousand dollars.

That bunch of applications on the Apple Store that gets bigger and bigger? They add up. You'll clench your teeth and try to ignore the constrains.

With Google you go here http://www.dataliberation.org/ and get everything out. Hell, you have the source for android, you can build your own system http://www.idroidproject.org/ and use your apps as well.