I recently spoke to someone that used to be part of the Fusion 360 team, and asked him about this. I was told that the intent is to provide Fusion free to hobbyists, indefinitely. Take that for what it's worth.
And having said that, when Autodesk offered an option (a few years ago) to lock into Fusion Ultimate for $300/year for life, I took them up on it, just in case at some point, it wouldnt be free.