The value of the stock isn't really in the cash value, but the company's tech, contracts, distribution network, and workers.

If all the employees quit tomorrow, he'd still be unfathomably wealthy from other sources, but the stock price would be quite a bit lower. I wouldn't call that "arbitrary" regardless of hundreds of acronyms you can slap on a stock chart (P/E is nearly meaningless because of accounting rules)

My thoughts of your point though - whatever the share price is it will always be too far out of reach for the employees to gain ownership of their own place of work. Whether its worth 1.5 trillion or 20 billion is a little silly to think about for any single person.

Irrational Exuberance is not the same as random nor arbitrary in the sense OP is using it. OP suggests that company values are conjured out of thin air, which both Marx and Buffet would laugh out of the room.