Considering how strong Yajirobe was with minimal effort, he'd quickly become the strongest Earthling if he trained properly. It still wouldn't be a game changer against the two Saiyans at first though and he likely wouldn't go straight into battle in this scenario, though everything changes when he attacks the wounded Vegeta as he'd have enough strength to finish him off. The Dragon Team still go to Namek and force Yajirobe into doing so too. With the absence of Vegeta to destroy Zarbon's scouter and Muri likely not having time to strike his, the group have a far more difficult time hiding from Freeza's forces when also trying to get to the Grand Elder and it's only a matter of time before they're killed. Goku defeats the Ginyus when he arrives, but even if avoiding getting his body stolen, he'd be underprepared for Freeza and have no knowledge of the Grand Elder, leading to Freeza killing him after transforming. Freeza continues to rule the universe unopposed for many years unless encountering Babidi or if he visits Vampa and makes Broly angry.
Meanwhile on Earth, #17 and #18 ravage the planet unopposed 4 years later. Decades later, Cell acquires enough energy to absorb them, becomes perfect and heads out into space. He likely finds Freeza and kills him. If he comes across Babidi though, regardless of the whole Cell VS Dabura debate, I'd say Dabura's magic and Babidi's mind control would be enough to catch Cell off-guard and make him Babidi's slave. Due to the now 26 planets remaining with sentient life

troll), Boo isn't revived though.