If nobody wants to buy your BTC at $10K, you don't get "your" money out, until coinbase can find a buyer to match with your sale.
In your example, all 10 of you start lowering your asking price, and "the market" goes down until you make a sale.
Coinbase may provide some liquidity, buying your BTC at "market price" with their cash reserves, but only if they think they can resell it at a higher price.
The risk they take on in doing that, and the liquidity they provide, is the nominal economic reason you are paying their fees.