You're comparing the valuation of horse and buggy companies in the 1910s to Ford.
You mean there are no other companies selling electric vehicles? And clearly, if, say, Mercedes, doesn't have electric model now, it's absolutely impossible that they have one in 5 years, just like a horse & buggy company, right? But forget about Mercedes, how about Nissan Leaf, which exists now and is the world's best selling electric car? Shouldn't Nissan be valued at similarly crazy valuations? Or they won't be part of this new brave world and don't get to eat from the "new slice of pie"?
Tesla has just changed the world, whether you realize it or not, and tables have turned against the petrol industry. The other brands have long bet the electric cars were impossible, they belong in history books together with our other guilty actions like inquisition, native americans and slavery.
Anyway, there's the obvious notion that capturing pollution from one central energy source (a coal or gas plant) is far more likely to reduce pollution than trying to reduce pollution from every car.
But you -did- say that Tesla is working this "completely new slice of the pie". Which they're not. Unless you artificially constrain it to a segment where only Tesla exists. Whilst the current Tesla models might well be "luxury electric", the upcoming stuff is entirely in the same segment as Nissan, hence the reply to you.
Tesla is not just a car company. They are going to be the largest li-ion battery manufacturer, distributed energy provider(with the solarcity acquisition), energy storage provider, supply grid stabilization services(important when more of the generation is not base-load), and compete in the autonomous car sector.
Its fine with me if people don't think those other areas are going to be profitable for Tesla, but thinking of them as just a car manufacturer is kind of missing the point. And those other areas I mentioned do make up a largely untapped pie that Tesla is poised to tap into.
Energy storage provider is "battery at a bigger scale", similar with supply grid stabilization. And as far as I am aware the PowerWall has been a commercial failure, even among Tesla owners. I'm not sure how that will magically do a 180 now that Musk is going from "influential adviser" to "owner".
There is the other side of Tesla Energy, which is the Powerpack. The Powerpack is the commercial/utility-scale product, available in quantities suitable for grid-level storage. They had over $1b worth of orders[1] for the Powerpack as of late last year, far more than they are capable of manufacturing currently. The Gigafactory opened slightly early to start producing those packs using Samsung cells, but eventually those will move to Panasonic cells as well. There is huge untapped demand just in batteries here that is only starting to surface and be fulfilled.
Grid supply stabilization has more to do with smart grids/smart inverters applied to solar panel + battery installs. SolarCity has detailed it in some of their more recent town halls[2].
1. http://fortune.com/2015/08/10/tesla-grid-battery/ 2. http://www.ustream.tv/recorded/87401050