Licensing songs costs money. The Pro is cheaper, so it got costs cut. I highly doubt you're going to see more songs added in an update, that would mean Stern paid for an increased license for the game you already bought.

The Pro is meant for location, where song variety isn't as important. If you're at home and want the full selection pay for the Premium, which is the home model.

Not saying I agree with all that necessarily, but that does appear to be the reality.

Why would you think that? These are all famous and iconic songs, which is why they were chosen. You think Sony just gives that stuff away?