Yes, on the one hand TPG certainly sold out too early. But they also turned TPG into a dominating brand and the first site that comes up on a Google search, which was unlikely to happen on its own.

Randy surely has some data, but extrapolation isn't easy and there's no way he would know anything more than ballpark figures unless bloggers share their numbers with him. Cards can pay anywhere from $10 to hundreds of dollars. Conversions are run hot or cold based on the whims of the bank, the quality of the applicants, etc. Sites that target newbies that don't have their own referral links and aren't hindered by signup bonus restrictions or approval frequency throttling restrictions convert at different rates that for sites that target knowledgeable users. Etc, etc.

Also, anyone can see ballpark traffic figures with sites like SimilarWeb, though affiliate revenue extrapolation from that isn't worth anything due to the reasons above.