Market capitalisation is the most useless number imo. All that really matters is revenue and profit margins.
Valuation only comes from what someone is actually willing to pay for a stake in the company. Everything else is theoretical until then and even after that.
Companies can and do go bust despite the best teams and investors. Stock market and startup ecosystem is not immune to any of it.
The market can remain irrational for as long as it likes, even if it's disconnected from ground reality, simply because it can afford to. There's enough money floating in the system.