Mercedes & Tesla trade at the higher end, where just the impact on perceived quality (in addition to the quantifiable financial benefits of buying a car that will maintain value) is important. OTOH, EV are already knowingly buying a car that ranks poorly on that front. Who wants to buy a 2015 model EV in 2018? Range improvements alone would kill the price.
So... not sure it matters too much yet. Going forward.. who knows. I think tesla may already be "in for a penny, in for a pound."