After gathering everything useful and making sure no fish had slipped through the net, Yinsen and the others set off on their journey back to Earth.
However, Vegeta did not travel with them. He and Nappa left separately in another Disc-shaped Spaceship, off to cause trouble for those small fry.
Yinsen felt that once Vegeta's strength reached a certain level, there was a good chance he would come to Earth looking for trouble. Of course, that was assuming he did not run into King Cold and get killed by him—Frieza's subordinates did not even know King Cold existed. Only Frieza himself knew.
Without a Gravity Chamber to assist his training, it would not be easy for Vegeta to transform into a Super Saiyan. He might get stuck at a certain stage and find it difficult to improve.
And once they returned to Earth, Yinsen and the others would soon receive a warning from Trunks, who came from the future—actually, from a parallel space: before long, two incredibly powerful enemies would appear on Earth: androids! (No fog)
After learning this, everyone, including Son Goku, would train desperately to avoid defeat and death in their battle against the Artificial Lifeforms.
Three years... no, in the original story, about another year passed after the battle on Planet Namek before Son Goku returned to Earth, and Trunks only appeared then.
That made four years in total.
Four years would be enough for Yinsen and the others to raise their Combat Power by another great deal. Still, raising their normal-state Combat Power to the level of Android 17 and Android 18 would be extremely difficult, let alone reaching the level of the even stronger Android 16 and Complete Body Cell.
"Do I still have to rely on transformations?"
The trip back to Earth would not