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?