W-9 is the form banks use to collect US citizen client's social security numbers in order to satisfy their FATCA reporting requirement.
If you are current on your US tax and FATCA obligations and want to keep your account open, there is no reason not to return the completed w-9.
If you are not current on your US tax and FATCA obligations and want to keep your account open, consider regularizing your US fiscal situation so that you can return the completed w-9 with your US tax affairs in good order.
Form W-8ben, if received, should never be completed by US citizens. This form is for "aliens."