It is not wise to keep your Bitcoin in an exchange, because it involves a third party, and if we can remember Bitcoin was not created to have a third party system, therefore it is risky keeping your coins in an exchange, and even if it's necessary it's should be a very small quantity that if denied access will not affect you.
It is better to keep your Bitcoin in your private wallet than in an exchange, where you can have access and control of it whenever you want it.
What you said is not wrong, but it will be a bit inconvenient for people who buy Bitcoins in small amounts through certain exchanges. Because every time the purchased Bitcoin is moved into a personal wallet, it will definitely require a small fee which if added up repeatedly will also be large. However, for those who can afford to buy in slightly larger amounts, I don't think it will be burdensome enough to always move it into a personal wallet every time they buy it from a certain exchange.
I also do not recommend storing any assets on an exchange to investors, but it may not be suitable for traders because they are more active on the exchange. But the best advice for everyone without exception is to keep it in a personal wallet and this does not only apply to Bitcoin, but also to other assets if the amount is large enough.