That would be the default of this system if the brakes are still able to communicate with and provide power for the brake pedal. Basically if the brake system lost external power, it would get the power needed to run via scavenging it from the motors. Once the vehicle came to a stop, it would then loose power and the parking brake would then engage.
The slam on brakes case would only trigger if there was gross damage to the braking system, where you might have bigger things to worry about.
I think you've lost sight of the initial concern that was raised. There's still a common-mode failure of loss of control power.
Control power is different from actuation power, the latter of which is what you're addressing. And it's not "gross damage" because it's a common-mode failure, so you can't just ignore it like that.
To lose local regen braking derived power requires a failure of the cars systems similar in nature to one that would cause loss of hydraulic fluid or air in a traditional braking system.
It is trivial to derive control power from actuation power - at least enough for a few can-busses that run from each wheel to up-to-12 independent sensors on the brake pedal.
It's trivial if you're doing an opportunistic design, but it's challenging to match the safety and reliability of a hydraulic system while beating the cost.
The slam on brakes case would only trigger if there was gross damage to the braking system, where you might have bigger things to worry about.