Currently, aircraft re-base page checks the limits at the destination, and if over the limit, it prevents re-basing.
However, new and used aircraft can be sent to the base, exceeding the limit. The main check for the limit is when a new flights is assigned to an aircraft. If over the limit, it fails. That's all that is needed. The check on re-base should be removed.
The reason it is convenient to temporarily have more aircraft is if player wants to replace / retire aircraft. It could be done with move schedule.