I don't think there will ever be a solid line because buyers cover a spectrum, from those that like the motorcycle look, if not the price to those that want their ebike to look like a road bike, and everything in between. Much like cars - some like a rugged, agressive look, while others want a prius look.
Manufacturers focus on products that sell, meeting the varying wants of their customers.
Even when legislation tries to establish lines, watch companies blur these lines, because some buyers (maybe most?) could care less about those lines.