Just think about it. Frieza was the strongest being in the universe, striking fear into the hearts of all who crossed his path, in his weakest form. It took the most powerful Saiyan in the world unlocking a mythical and legendary transformation to defeat him. If a Human could have just trained extra hard for a few years to surpass Frieza, it would be a joke. Humans would be regarded as the ultimate warrior race, putting saiyans and every one else to embarrassing shame. For story purposes the human characters still get some screen time so they dont become completely irrelevant, but no, Krillin would never defeat Namek Frieza unless he unlocks some super ultra instinct zen human form that gives humans a massive boost.