I think people are reading this backwards. It isn't just using local storage and indexeddb to protect cookie values from being wiped (by the user), I can see it being more useful for using cookies as an extra store to protect localstorage/indexeddb from being cleared by the browser.

The user can still clear the data manually (clear cookies and other local data) or through a properly written app (which implements a "remove my records from this device" option, and preferably makes the local storage opt-in anyway).

If you are thinking of using this maliciously then you wouldn't use this: you'd use the already existing evercookie instead as it survives more (at the expense of potentially imparting more load on the users browser).