am pretty damn sure not all exchanges will be ready to Buy your bitcoins at current prices, and it did happen in the past when btc made it as high as 20k usd.
This may be true for low volume exchanges, but high volume exchanges may have enough buy orders.
If you don't find enough buy orders in one exchange then you can split into several exchanges to sell your quantity.
I guess, someone with large BTC hodling may face this issue otherwise all others may not face this issue... Please correct me if I am wrong.
There could be another solution, convert your BTCs for a stable coin for a certain time and then convert it to real money at the right time.