All this is a complicated topic.
In short, we have been working to try and fix serious problems that come from the legacy of 20 years of file based cad. There is much that has not surfaced in the tool yet. Assembly references, design variations, as built and branch and merge will solve pervasive design problems. We also know that we must interop with file based systems seamlessly. To make cloud storage optional would be to remove one of the founding technologies upon which Fusion was imagined. We would be looking backwards not forwards.
I complete agree that we have not always met the need of easy file and data management or performance for some users. Each update has internal and external improvements on this front. There will be much improving in the first half of 2015.
We are working on a much deeper explanation on how we think cloud based, decentralized project data can be the basis of powerful collaboration and design available to anyone. I'll post that here too.