Ultimately I think there's a strong case to be made that Canon will never phase out the EF mount, because honestly what do they gain by making a new lens in the RF mount?
Money, that is a simple answer.
They have more functions that they can put on the lens. People like those types of little extras. You have to entice people from using the old EF lenses you are talking about. So you add a few extra functions and people are enticed to upgrade.
FWIW, you can insert that answer in to anything where it involves a "Why would Canon...?" question.