I agree with Eddy that the weakness of the concept is that in Formula E is that they have to change cars mid-race, and in TT Zero this shows up as being a 1-lap race by necessity. However I know there is a serious amount of research going in to energy storage and re-generation etc so I imagine somebody, somewhere is going to crack this particular shortfall.
With regards to the original question I don't think Formula E is a threat to any potential TT World Series as these are both aimed at a very different demographic and so I don't think they are a threat to each other.
As far as 2-wheel electric bike racing goes the rate of technology development will be far more rapid with Formula E (and Formula 1) because they have access to more resources, financial and physical, and the results of this development will be accessible, and applicable, to bikes and so will help EV bike racing.