The problem with your personal data store is that it will be impossible to stop the large companies like Google/Facebook from simply copying and caching a copy of all the data they need to access. You give them permission to handle your email and calendar information and bingo, they have a copy of your entire set of emails and appointments. If you use multiple services from Google/Facebook then they will end up with…
The key difference is that instead of companies storing, owning and exploiting your data, the user maintains their own data store and companies can then access and exploit it.
It's decentralised in the way where the data lives and is controlled by, not in how or where the data is used.
There is the problem where companies could continue to use your data when you no longer let them access to your data store - but that in effect is at worse unethical and at best not legal these days.