In order to assuage some of the fears that people may be having about Stadia as a platform that will be worth investing in as a consumer, Stadia's Director of Product Andrey Doronichev in an AMA on Reddit yesterday likened the transition from a traditional gaming console or gaming PC hardware to Stadia to the transition of using devices like CD players to grabbing all of your music from streaming services. He further drove his point home by comparing movies and personal files or photos in the same way.
Doronichev's point with these comparisons was that people were apprehensive about these transitions too, but now they love the capability to stream music and movies and store their files in the cloud. Those comparisons aside, Doronichev offered up a bit of information that might make some consumers happy. Stadia will allow you to download your game metadata, and your game saves.
Now, Stadia will save all of your game data in the cloud, so you won't actually need to do this, but it could provide some measure of comfort in knowing that you can download all of your game save data and keep it stored somewhere locally should you feel the desire. This could also help in the event that there's an issue and your game saves are wiped out for any reason. Not that that's likely, but anything is possible and it's always a good idea to keep stuff backed up if you have the opportunity.
In regards to the metadata Doronichev doesn't mention exactly what metadata is being collected, but if you want to you can download that too for safe keeping offline. Again, downloading your game metadata and your game save files is something you can do as an extra measure of safety just in case, but all of that data will be stored on Stadia servers. Even if you quit your subscription and come back at any point, that data will still be there so you won't lose it.
Perhaps the only time you might lose it is if Stadia ever becomes a service that Google decides to let go of and shut down, in which case downloading your game saves won't do much good as you would have nothing to restore them to. They would serve as little more than a memory that you had gotten to a certain point in the game that they were pulled from.
That being said, Doronichev points out that Google is super committed to Stadia and that it has invested a lot of money and tech into the platform. The Stadia team as well has invested a lot of time into bringing this service to a place where it wasn't just a vision but is now a reality. Doronichev really believes in Stadia as a product and as a future of how we access and play games, and he says that Google nor the Stadia team have any intentions of doing anything other than continuing to invest in Stadia for years to come.