Can't you still run it downhill and regain energy? A lot of downhills in the city can't utilize the gravity all the way, instead having red lights just at the bottom of the hill
The efficiency gains of bicycle regen is not high enough and there is added drag of pulling a motor along if you are just pedalling normally. Most regard the trade-offs to be not worth it.
There is one Canadian ebike parts manufacturer that's pushing for regen, and their main reason for doing so is that it saves on brake pad wear.
And where is the heat dumped? Why does the physical resistance disappear when the battery is disconnected?
Of course there is the minor problem that the hub was mounted in the rear wheel so it's only good for gentle to moderate braking, but thankfully that is the vast majority of what you do.
That just means you had stored potential energy, otherwise how are you going to get up the hill?