Your definition is not common to the market. Users can’t possibly anticipate whether the phone will provide acceptable performance when it reaches this ‘100%’ state. The devices are reviewed based on as new performance. The device provides adequate performance for some time, subsequently stops providing adequate performance, without notifying the user that there’s a way to restore acceptable performance. This is extremely user hostile behaviour.

I would literally rather my phone started randomly turning itself off at 30% power. At least that way I might take it for repair, have the reason diagnosed, and get it restored to acceptable performance, rather than simply assuming that software has moved on and I have to live with a slow device or purchase a new one.