If someone invests time, money and resources into trying something amazing that has sound theory then even if they fail it is still an advancement and good for society. That theory can be scrubbed out and we can move on (with their money at least being invested back in "the economy").
This applies to all things like space flight, self driving cars and concept devices.
However those that sit on a theory and don't proceed due to risk aversion, cost worries or general apathy don't really help anyone, they might even restrict anyone else from pursuing it.
Maybe the risk will pay off and make a fortune, however it is better a private, rich individual take that risk than the government squandering billions.
With Elon Musk , the jury is still out on whether electric cars can actually have as much market penetration as he hopes and become the primary vehicle power source of the near future. It is great that someone it ploughing lots of money into it to really try it out and try to prove the sceptics wrong. If he fails then most can be satisfied that it wasn't through a lack of trying.