What % of total Coinbase BTC could be withdrawn?

So I mostly just joke around on /r/Bitcoin and jokes are fine, but the first time I asked a serious question there it was mysteriously censored. I must have touched a sensitive nerve. I thought people here might have more critical attitudes.

The question is about Coinbase. Personally I wouldn't touch it with a long stick, but it seems like most of the general public buy and sell BTC there. They also have wallets on Coinbase.

But I hear nothing is done on-chain there. So what are the consequences of that?

In the fiat world we know that if 100% of bank account holders asked to withdraw their money, it would be a disaster. Or even 10%. The money doesn't exist.

Analogously what would happen if all Coinbase customers suddenly wanted to retrieve their BTC to private wallets? What percent would succeed before there was just no more Bitcoin to give out? Please estimate. All hypothetical.