Expect an iOS 10 Downgrade
If you decide to download iOS 11 on release day and you decide that you hate it and its performance, you should be able to drop back down to iOS 10.
Apple will continue signing off on the iOS 10.3.3 update through the iOS 11 release which means you'll be able to take advantage of a loophole and drop back to the previous version of iOS.
Now, this loophole won't be open forever. Apple will close it up and it will do so without any warning.
Typically, Apple signs off on updates for a few weeks before stopping. Once it stops signing off iOS 10.3.3 there will be no way to get back to iOS 10.