A bubble that never bursts is indistinguishable from real value. Remember that money is itself a bubble; its only value is that you believe other people will continue to accept it to give you the things you really value. The dollars in your pocket are just pieces of paper; the dollars in your credit card are even more nebulous, they're bits and bytes in your banks' computers. And yet somehow it's worked for thousands of years.

Plus, it's pretty likely that several of the companies listed will go public in the next 5 years, and then their valuation will be the public co $X. If that's lower than the stated figures in the bet, well, Sam will lose.