The differentiator for me is battery weight. Mazda can't make an electric MX-5 Miata with current battery density as it would cease to be a Miata. One of the core improvements (in my opinion) between the 3rd and 4th generation Miatas is the diet it went on, and that was only 110 lbs in the 2.0l model. The Japan-only 1.5l lost over 300 lbs over the 3rd gen.

Consider the Lotus Elise and the original Tesla Roadster. The Elise weighs in around 2,000 lbs. The Roadster is around 2,800 lbs. It's not 4,000 lbs, but it's a good chunk o' change more. I think we'll get closer eventually.