The first thing is to expect the app devs to define a api in RPC so that what they app serve it transparent to the whole platform.
That way, users and other apps can reuse that api and integrate tools and other services without worrying about if the node is online or not.
Given they are RPC (over TCP or IPC), they can be used as a peer node in a distributed computing flock or in the app service process the application can serve the RPC requests not locally if it wants or needs, but going over the network.
But even in the case it goes straight to the "cloud" for resources, in the node it will always go through the RPC-api first. (How the application handle the RPC request is defined by the app developers).
There are a lot of others important details actually, for instance the UI sdk, window management, its already there, the storage layer (files and key-value db) are distributed over p2p(torrent) and everything is accessible/bootstrapped through a DHT address accessible anywhere.
But imagine if you had this architecture before, and Twitter, Facebook or Google search had to install a api-based application, where they need to ask you for permission to index content, or store locally your list of friends.. where later other apps could extract that information from your machine.. For instance you could change your search to DDG or export your Facebook list of friends and post to some other social network(this is actually the primary reason almost no one could compete with a popular social network).
In the end, things that should be yours are yours and third-party apps will have to be installed and granted permission by you to manage that digital property.
But overall i'm very glad to see that this is starting to become some sort of a trend, that people are finally waking up, because despite of all technological implications of it, it also have a lot of political and social benefits to this approach.