The i3 was just as successful as many cars on the road. Again, you are making statements without defining the criteria you are using to make your statements. What metric are you using to say it wasn't successful? Profits, production numbers, years in production...?
By your own admission, the system can be used to improve an already balanced system with minimal weight (active diff on the M3 is like 10-20lbs heavier). You suggested EVs are flawed, so they are using the torque vectoring to fix a problem, yet they may be just taking advantage of something inherently available. The skateboard design has a low center of mass; the battery is center, so it can have a good polar of inertia for rotation; the motors are between the drive wheels, so weight is directly over the wheels for traction; a front and rear engine can create a near 50/50 weight distribution. This is all good stuff, or even better stuff.
There are variants of the same car optioned without the active diff, and they still destroy older variants.
Because you said the i3 wasn't popular, so by your own statement someone would reasonably conclude you care if a car is popular. Seems popular enough if it is going to be in production for ten years and sales have gone up each year.
https://en.wikipedia.org/wiki/BMW_i3#Production
Again, the point of the BMW i3 is to demonstrate that EVs aren't necessarily heavy. A different form factor version of the BMW i3 in a sports car format could be even lighter. Range and speed aren't necessary for a sports car. Going to the track is a consideration for enthusiasts, but not really on the list of concerns when producing an affordable sports car. The vast majority of Miatas and 86s never see a track or get a single modification. I don't see why an EV would be different.
An EV Miata is totally doable with today's technology and still be compelling. It might even sell alongside an ICE version, and it could do well, especially at autocross or for short tracks, but it would be even better as a no-fuss weekend Sunday driver that would need little maintenance. For anyone in a city like me with a 3 mile commute, and who is 30 minutes from the coast and who has a two hour loop for canyon driving, 150-200 mile range is more than enough.