Eventually at some point, you have to get some work done, and you just can't do that with a tablet screen or a touch interface. The mouse and keyboard will reign, and even if the concept of a non-sanboxed system goes away, the future will still have hardware which look like desktop computers and laptops. The OS filesystem won't go away because applications on these personal computers need to be able to talk to each other somehow, and the current iOS method of "launch a file in this application from this other application" is too limiting. The only thing that will change is the application packaging, which will become optionally centralized and have permission levels just like Android apps. I think overall, future computers will be just as pleasant to use, just as programmable/configurable, and just as easy to download software from decentralized sources.

Just think, Chrome OS has essentially failed to take over the OS market, so I doubt anything using this overly-sandboxed model will be successful in the future.