We've officially reached the last stage of the tech unicorn boom

On Wednesday morning, the kingdom of Saudi Arabia invested $3.5 billion into Uber.

It called to mind what an investor told me about a month ago when we were talking about the end of the current boom in Silicon Valley and what happens next.

This person didn't want to be identified, but he's one of the best-known venture capitalists in the valley and has been involved in the early stages on some companies that went big.

He told me that a lot of companies would deflate and be unable to raise another round without revaluing their stock. But he doesn't expect a sudden bubble pop like in 2001 because there's still so much money flowing into venture capital, especially from overseas.

As he put it:

It's so amazing how much money is available on a global scale. It's unprecedented ... I speculate - and I qualify with that word - there's a lot of money in Russia, the Middle East, and China that's looking for country diversification. Or in the case of Russia and Middle East, it may be looking for sector diversification. With oil price shocks, if you're a Saudi prince, do you want all your money in oil?

As this money continues to flow in, the unicorns will be able to buy more time.

But eventually, many of them won't be able to repay those latest investors, and they're going to get soaked. In fact, investor Bill Gurley had a warning for these late-stage investors last month as he predicted the end of the unicorn boom:

It's not the second inning or even the sixth, it's the fourteenth inning in a five-hour baseball game. You are not being invited to a special dance, you are being approached because you are the lender of last resort. And because of how we meandered to this place in time, parting with your dollars now would be an extremely risky move. Caveat emptor.

Gurley was also on CNBC on Wednesday morning - before the Saudi deal was announced - saying something similar:

The one thing I would remind anybody who's thinking about getting into late-stage unicorn investing right now: You are the lender of last resort. They are coming for you, so pay attention. Caveat emptor.

Caveat emptor = buyer beware.

Worth noting: Gurley's firm, Benchmark, was an early investor in Uber, and he almost certainly wants the company to go public so it can realize its gains. He's not a disinterested party.

Still, this feels like the final stages of the current unicorn boom, like when your best friend who never invested in the stock market started buying dot-com stocks in early 2000.

The good news is that Silicon Valley always renews itself - for the 50 or 100 unicorns that don't live up to their promise, one will become the next Google or Facebook and there will be dozens of newer and more promising startups following in their wake.