Not just for take-off, but for the possibility of finishing the take-off with one engine failure. That's quite a bit of spare capacity. However I also wonder w

Not just for take-off, but for the possibility of finishing the take-off with one engine failure. That's quite a bit of spare capacity. However I also wonder why, if electric motors are a solution here, there hasn't been a comparable fuel-powered extra-takeoff-engine. Why would this not have the same benefits? Or are extra engines simply too complicated mechanically unless they are motors? (The liked article is about modifying a Dash 8, which is a turboprop, and perhaps it's easy to have oversize propellers for takeoff, and just connect a motor to the same gearbox.)