The % of people that will actually buy one because they are of ill health and can't spin anymore is minimal. Somebody suffering from bad knees could get an aftermarket electric motor fitted to almost any bike for far less than the cost of a whole new "E Bike".
I know this because a friend of mine used to have a relative who did this, and as mentioned before they can be manipulated to run much much faster then legal/safe. He has maxed out on one at 62mph on the flat.
This whole fad is just the bike companies milking you for cash, next year they will release new more powerful/lighter/cooler/must have "E Bikes" and some of you poor buggers will be mugged into buying it.
As mentioned before, I'm not talking about Doris & Frank popping down to the village shop for a pint of milk and a paper, I'm all for them using a power assisted bike rather than a car but just look at Cube's new website and the F/S Hybrid bikes.