You're implicitly referring to the subset of end users that have chosen or would like to choose devices from incompatible ecosystems.
I suspect this set of users is relatively small, given that it excludes what seems like the most common case: people that are satisfied with the platform they're on. I don't know that there are a lot of Android users (for example) that are eager to buy iOS devices but hesitate because they won't be able to use their existing apps.
I think you're wrong. Especially going forward, how about apps that run on your TV, your tablet, your phone, your computer, etc. Want a chromebook? Runs there. On a friends phone? Load it there. What happens if a platform dies out? Or another becomes significantly more attractive to you because they release a poor maps application? What if you want a cheap tablet for your kids? There's a ridiculous amount of use cases that do matter to most people. The reason you're happy with your platform is actually because they've locked you into it. If another closed platform came around with all the new desirable apps, you'd be unhappy for the same reason you're happy about it now.
The reason you're happy with your platform is actually because they've locked you into it.
I don't agree. Lock-in is when "I want to switch, but can't." Happiness is "I don't want to switch." Most users on most platforms are happy or at worst indifferent to the question.
I don't think it makes sense for most developers to target the scenarios you describe because although they are numerous, each is small. I don't think the edge cases add up to more than the common case on any major platform.
(Disclosure: I don't regularly use any smartphone, tablet, tv or anything that could be called a modern web application, so it's not my happiness I'm talking about.)
This seems like a bad assumption, and a worse one going forward.
And right now, I end up having to find the right device to run that app I want to use instead of using the device that I like with the app that I like...
No, I'm not. I said "platform". A single platform can encompass multiple devices.
An Android user that likes Android is less likely to decide to buy an iPad than he is to buy an Android-based tablet, on which is existing Android applications will (generally) run. iOS is much the same, and Microsoft is seeking this across desktops and tablets with Metro (or whatever they're calling it now).
Even within a single "platform" you can have apps that are available on some devices but not others. It's not that hard to find Android apps that are only available on phones, not tablets, and vice versa, or for that matter ones that work on some phones but not others.
I suspect this set of users is relatively small, given that it excludes what seems like the most common case: people that are satisfied with the platform they're on. I don't know that there are a lot of Android users (for example) that are eager to buy iOS devices but hesitate because they won't be able to use their existing apps.