I have an acct from the dawn of gmail and I'm still only using 25% of the current quota. I don't know the exact numbers but I imagine it's basically the case that 99.9 percent of inboxes never run out, and a significant fraction of those who do are abusing it with gmailfs or something.
Gmail now being a for-profit, floated business isn't the point of it to target people with advertising. The function is only to achieve the rough aim of causing people to buy products they otherwise wouldn't have.
This isn't an unusual mode of commentary for HN, but one of my resolutions is to point out when social cynicism has far out-stripped reality.