The Saiyans won Freeza because he was a lazy bastard, although Saiyans don't have an average power level anywhere near Frieza's a few talented ones like Goku and Vegeta had potential to achieve and far surpass the level of power Frieza had naturally without training at all, but guess what, when Frieza decided to train only 4 months he did far more progress than Goku and Vegeta did on decades of training, he even achieve a power compared to the gods, maybe a human can achieve a higher power level and in fact they just achieved it, Krillin is maybe +120,000 that's Cap. Ginyu level and that's probably stronger than 99,9% of the universe 7 now, if they tried to train like Goku and Vegeta(higher gravity and time chamber) maybe they could achieve 1st form Frieza level after many decades of training but the reality is that they never had or tried to train like Saiyans because it is impossible for them, they're only human, they would never be capable of training like Goku and Vegeta, their bodies would never resist that kind of harshness and that's probably why Goku never taught them the Kaioken, they can definitly progress but you can't expect them to get to the level of even Namek saga Frieza, maybe that would be possible only with an After Life body and only after many centuries of training just like that filler character Olibu and that's it, a pretty fucking strong human but still nowhere near Saiyans, Nameks or Frieza.