I am 99.9% sure it will never become a pay service as Yahoo! would lose ALOT of users. Of course, they have every right to do so, but the chances are slim to none, especially since they already have ads everywhere.
This goes along with any IM's you may have received saying Yahoo! is shutting down accounts that don't foward the message. (Or that they will start charging you a fee.) Wrong! Don't believe and don't foward these messages. You help reduce the amount of endless spam/spim that is already circulating around the internet.
