I get your point, but are any of these companies Palo Alto based?
Nope. But a lot of their workers are. And those are the ones that are bringing the revenues. Spending their paychecks in the city. Paying property taxes, utility bills. The companies themselves usually don't pay a lot to a city directly. It's all indirect.
That's why I'm saying: remove the 10 biggest tech companies and Silicon Valley will become a silicon desert with a huge housing crash.
Just take a look at Detroit to see how quick a booming economy can crash.