The number of companies that have bad unit economics is, I agree, substantially higher than it should be. But I'm not sure this explains the bubblegum expansion of valuations and rounds we're seeing today. There are many thoughtful comments on the perceived bubble from people all over the industry. I'm not qualified to say one way or the other, but I did enjoy Mark Cuban's recent post on the matter. [1]
My company, Thumbtack, got a lot of this right early on. Years ago we doubled down on getting the unit economics right. So right that we were (IIRC) profitable for a month or two before taking a big VC round. Not sure how many other companies can say that. But at the end of the day, if the bubble is a bubble and it pops, knowing that we would survive and be able to make a profit is hugely satisfying.
My advice to any young startup is figure out how to make money early on. It's the rare Google/Facebook that can blow up and use the platform to sell ads. Most startups will not do this, but many can still make good money doing important things that people will pay for.
[1] http://qz.com/356620/mark-cuban-is-absolutely-convinced-we-a...