Whilst you are technically correct, I don't think this is what most people would have in mind when they are thinking of "open" systems.
Have a think about the types of application that are trotted out when people want to point out the advantages of open android :
a) new virtual keyboards - not possible with sandboxing
b) new launcher screens - not possible with sandboxing
c) access to data from other apps - not possible with sandboxing
Or rather, if you can do these things with sandboxing, then sandboxing is not providing the protection that you need. How do you guarantee that the funky new keyboard you just downloaded isn't actually a keylogger sending all of your passwords to a server somewhere for nefarious purposes. Or less nefariously, how do you know that the new keyboard hasn't forgotten to implement a special key that is necessary fo you to use application X. How do you go about backing out that installation of the keyboard, including the reinstallation of the default keyboard?
On the other hand, if you protect against these problems of security and confidence that an app won't break the system, what exactly are the advantages that such a system would have compared to a closed system such as Apple provides?
Did you ever stop to think that, despite this being a potential problem, why it has not been a problem?
Android isn't an immature product. google have activated something like 300 million devices. 300 million opportunities for your theoretical issue to manifest and yet it hasn't.
No, of course I didn't stop to think, that would just be a waste of my time - much better to just send off a response without any thought what so ever...
Of course, back in reality, I do actually know someone that installed Swype on their Android phone, didn't like it, and couldn't figure how to get back to the default keyboard, so I don't know why you think these types of problems aren't already cropping up in the Android world.
New keyboards have to be manually activated in Android, via the same mechanism that you'd use to switch to the default keyboard, so that honestly sounds like a pretty straightforward PEBKAC (heh) problem.
Turning on Swype implicitly trains you in how to turn it off.
Have a think about the types of application that are trotted out when people want to point out the advantages of open android :
a) new virtual keyboards - not possible with sandboxing
b) new launcher screens - not possible with sandboxing
c) access to data from other apps - not possible with sandboxing
Or rather, if you can do these things with sandboxing, then sandboxing is not providing the protection that you need. How do you guarantee that the funky new keyboard you just downloaded isn't actually a keylogger sending all of your passwords to a server somewhere for nefarious purposes. Or less nefariously, how do you know that the new keyboard hasn't forgotten to implement a special key that is necessary fo you to use application X. How do you go about backing out that installation of the keyboard, including the reinstallation of the default keyboard?
On the other hand, if you protect against these problems of security and confidence that an app won't break the system, what exactly are the advantages that such a system would have compared to a closed system such as Apple provides?