I believe your guess for frequency is off by a lot, my guess would be more in the vicinity of a tenth of those assumption.

However, more importantly: going in with a "we need to support this" mindset alters incentives. Suddenly, useless error messages and lacking documentation aren't okay any more.

A different approach to solve the issue, if it even exists at that scale: make them pay for it. If you want service, pay $10. That's more than enough cover Google's cost and it's cheap for anyone that really requires service because e.g. their account has been locked, or Google's systems have disabled ads on their site. At the same time, it's expensive enough that you don't get flooded with "why do you show me those videos on the front page?" requests.