That would only be true if you think that those industries have maximized all potential improvements based on technology, distribution improvements, and scale.
So, no.
As for that quote from Elon, is there actually a source for that? The car was being adopted long before Ford came around, and there was high demand, it was the mass production methods that had it take off for the mass market.
If there were a substantial number of people who didn't want cars, the industry never would have taken off.
I am sure that there were some people who were negative about the ideas of cars, but it is a bit silly to think that it was prevailing wisdom given how much it did take off.
Besides, how is giving Jayden Smith and DeCaprio $7500 off their taxes when they buy a Tesla making things cheaper in the long term?
RobStark as already said part of what I'd say, but it also has to do with developing the technology. Whenever the world has some kind of new technology, there comes a stage where it needs to be in the hands of the public to work out the final bugs as well as develop better mass production techniques. No matter how you slice it, it was going to be expensive to develop affordable electric cars for the masses. ICE are a very well established technology with a support network optimized for them. Electric cars need a very different support chain as well as being a different technology under the hood.
The early EVs were inevitably going to either be undesirable cars with too many compromises or very expensive cars. Most car makers went with the former while Tesla went with the latter. Several years ago Elon Musk laid out the Tesla plan: build a car that proved EVs could be fun and have decent range (the Roadster), then make a more affordable car that was more practical (the Model S and X) and finally make a car that can be mass produced that is as good or better than an ICE and is affordable. Tesla has stuck to this plan and the Model 3 is the final step.
The Model S and X were never going to be mainstream cars because of their price. The only people who were ever going to be able to buy one were the rich or those with enough disposable income to stretch their budgets that far. At minimum you're looking at 1%s or top 10%s with small household budgets. To learn the lessons needed to be able to make the next step to the affordable car, Tesla depends on those tax credits to the wealthy to sell enough cars so they can learn the lessons needed to make the Model 3 as well as make enough money to afford the Model 3 development.
If the government instead had just given grants to car companies to develop electric cars, I doubt Tesla would have gotten as much of the pie as they have and instead all car companies would have taken the money and cranked out more heavily compromised EVs with poor range and not very popular outside the extreme eco crowd. It's other incentives that sell Leafs and Volts. I think there was a survey done of California EV buyers and a large percentage bought one to get the HOV lane sticker which allows them to drive alone in the HOV lane on the freeway. Among non-Tesla buyers, most of the rest were eco car buyers or people trying to avoid the high price of gas, which is not much of a reason these days.
I'm not a big fan of trickle down economics because it depends on those benefiting passing on a fair share of those benefits, which usually isn't the case. In this case, the rich getting the tax breaks aren't passing on their benefit, but Tesla, who also gains through increased car sales, is passing on this benefit by making a more affordable car that the rest of the world can buy. So this is a rare case where trickle down economics actually did work.
The incentive is working some some degree with other car makers. GM is introducing the Bolt next year, which is behind Tesla in many ways, but it is ahead of anything else the mainstream car makers have built thus far. There have been a number of announcements of next generation EVs from various car makers, some may make it to production.
Cars are a lot more complex technology than solar panels. Giving incentives for those is pretty easy to see, give a home owner a tax break and more people will put them on their houses. It's more difficult to see the benefits for cars, but they are there. A lot of things in the car industry have to change to support EVs both on the consumer end as well as in the design and construction. The Tesla skateboard battery pack is beginning to be adopted by other car makers because it's a brilliant redesign to take advantage of the new fuel type. Nobody has yet adopted Tesla's motor placement though, which I can't quite figure out why. It may be due to hidebound car execs who want to see an engine when they open the hood.