Why didn't they think of it?

             Why didn't  the Poskim think of just banging without announcing the new Nusach?

Perhaps the purpose of announcing יעלה ויבא- ותן טל ומטר etc the first מעריב it is said, is not to remind people to change to the new Nusach.

The reason for the announcment is to make it official for the new Nusach. By announcing  יעלה ויבא- ותן טל ומטר etc we declare that from now on  we change to the new Nusach.The same way when we Bentch טל &גשם it is not to remind us to change but it creates the change because of the הכרזה so too all other הכרזות create the change in the Nusach. Therefore banging on the Bimah without any announcment serves no purpose and none of the Poskim even mention doing it.