I see your point, and I can see some application in group rides, social events with a less fit SO, but
1) in TT, or even in any regular bike climbing, holding constant power is an important skill to master. Try doing a 45min varying gradient climb at 5-15% below FTP. It is not only your legs, but also your brain anticipating the gear shifts. Give the Alpe du Zwift a try, turn ERG mode off, and you'll understand what I mean. Variance in the power output wears you down. (that's why we use Normalised Power numbers for fatigue). You've to train that, not by doing3min on-1min off intervals, but by riding on real tarmac. Also, after few thousands hours of training you don't need to look at your head unit to know if your power is constant or not.
2) Power is not the only bottleneck for riding 300W, 400W, 450W for 10min, that where the handling skills of your bike come in play. That one of the reasons why we do 'speed training', cycling behind a buddy on a scooter/car. To get used to the cadence, braking, steering, ... Imagine participating in a race, riding 50+km/h without any fast cornering/breaking experience, that's a hazard!
3) Imagine doing an all-out interval session on TT bike, in a full aero position, and getting passed by an elderly on e-bike. Everyone will think "that guy is a bit weird", won't they?
4) Max. power output of this motor is 250W, I believe this to be an EU regulation. That is 20% less power on the rear wheel than the FTP of an average amateur racer. There are also speed regulations in EU, 25-28 km/h. That's certainly the Strava numbers to show off! You can make it a motorcycle/scooter and remove these limits, but then you'll need to wear motorcycle helmet in many EU countries. Also, you are no longer allowed on the bicycle lanes.
5) People who are serious enough about their cycling are generally quite serious on the bike position. For example, the Q-factor is the distance between your shoe cleats. Favero released power meter pedals for Shimano SDP-SL cleats, but added 1cm to the q-factor, just look at the flame war it caused.
6) Most people train on their 'race-day' bike. Some have a spare 'bad weather' bike. Now, you want to train on an additional training bike, with a different geometry to the race-day bike? Or are you expecting to have a TT bike with this drive train + a race day TT-bike? TT-bike will run into 5-15k USD, I might like my hobby, but won't put extra 5-15k for this nice to have feature.
I think 'niche market' is an underestimation, since there are already assisted racing/MTB bikes (limited to 28km/h), just slap power meter pedals on and you're ready to go.