Also imagine being on the subway sitting right near the door to the train and when the train stops you get off but your seat was right by the door of the train so you're still within BT range when you step off the train. The train leaves and once your iPhone understands that it's no longer connected to their Airtag via BT THEN it alerts you. The train has been long gone. This is why your idea doesn't work because the execution will never be ideal. Furthermore people will try and blame Apple for the alert not happening fast enough to prevent them losing their keys.Imagine being on the subway, getting up with your keys on the seat, you walk to the door as your stop is approaching and your iPhone tells you right then that you're separated from your keys.
I don't understand why people keep coming up with these less than ideal scenarios. The Airtags are only $30 USD. At that price they should be used for simple and ideal situations.