Without their multi-trillion dollar valuations the U.S. national debt would become unsustainable pretty quickly. Investors look at the potential profitability of a nation to decide whether its debt can be financed.
So be careful what you wish for!
Whether or not we have a google, we will have advertising, perhaps at more competitive rates with more competition. Monopolistic Megatech v. No Tech is a false choice.
On a straight revenue side, corporate tax, in total, accounts for about 10%. Granted there is additional income paid through salaries because of these giants, but the I doubt with reallocation it is profound as you are claiming.