In a decentralized system, where immutability guarantees are in place, re-encoding is not possible regardless of clients willingness to re-encode.

As far as battery life is concerned, its like playing a 3d game. During encoding, a fair amount of battery power will be consumed, so you're probably going to want to plug your device in for long clips. Typically, you will also want to upload long clips on wifi, likely near a power source, so I don't see where is the problem here.

Most streaming/video apps already do partial, if not full encoding on the device.