Hypothetically speaking, if every user were to fill their 2+ gig mailbox quota, would Google actually have the harddrive space??
I have no first-hand knowledge of this, but it’s my experience that with most services, they keep storage on hand for likely scenarios, and somewhat unlikely but plausible scenarios, but never for wholly impossible scenarios like what you described. It’d be hard to make a business case for full drive capacity when you probably only need a fraction of that.
Who’s gmail and what’s her number? I’ll let you know in the morning. 
I highly doubt they have that much harddrive space.
Similarly, banks wouldn’t be able to cover it if all thier customers tried to withdraw all their money all at once. But, like this scenario, that would never happen.
Agreed. It is the same concept which allows airlines to overbook. And it is this example which has the most risk, ie. sometimes every single passenger does show up for a flight, costing the airline money in having to bump folks or put them up in hotels etc. Still even on the narrow margins of luck the airlines use in this case, it must be profitable for them overall as they keep doing it. In Google’s case, OTOH, overselling must be much, much less risky. They can give over space from light users to heavy users, whereas the airlines can’t deal in fractions of a seat.
Um, actually it has. 1930. The Great Depression. People panicked and started runs on banks, enough to where many banks were forced into bankruptcy.