> what would otherwise be IPOs or other public financing rounds

The NYSE and Nasdaq do a lot of hype about how much money they raise for companies in IPOs and secondary offerings. But if you look at the numbers since World War II (and even before), very little of the money corporate America raises comes from the stock market. US corporations raise money via bonds, term loans, credit lines and so on. The stock markets facilitate stocks moving from one shareholder to another, but have (relatively) little involvement in raising cash.

As you note, companies are staying private much longer nowadays. Facebook didn't go public until it was worth $100 billion.