I think the sort of mastery that makes the Sen'nins sen'nins differs from the training that makes Goku capable of leveling mountains with a fart.
It took KameSen'nin 50 years to master the kamehameha, and three years to get the water from Karin, while Goku did both of these in a tiny fraction of the time. Even though Kuririn and Yamcha took longer to accomplish these, they both did it more quickly than KameSen'nin.
In Goku's case, well, he's Goku, and a Saiyan to boot. I tend to think "energy" control is an inherent advantage that saiyans have over Earthlings, so Goku's instant Kamehameha isn't all that special.
Kuririn had been at a monistary before training under KameSen'nin, and he and Yamcha both watched KameSen'nin and Goku use the Kamehameha before they used it--and I'm not sure I'd say either learned to control it all that well.
Meanwhile, KameSen'nin blew up the moon.
I think KameSen'nin trained to master ki, while Goku and Kuririn trained to master martial arts. If Kuririn hadn't settled down to start a family, I could see him gaining Sen'nin status.
T'en and Chaozu... I could see either of them to be 'immortal' in the Sen'nin sense by the end of DBZ.
I don't think Goku cares. Hee.
I'm not sure what to make of TaoPaipai, though. Perhaps Tsuru-Sen'nin kept reinvigorating him until he got enough control to keep himself alive for centuries?
