well, I hope universe 6 has more to offer than stereotypical "evil" versions of our heroes. If thatīs the case, all weīd have to look forward to would be boring fights against evil Goku and evil Vegeta, because thereīs no reason to assume that thereīs anybody else on their level. Also, if Goku never left earth, then there shouldnīt even be a super saiyajin. That only came to be thanks to the whole plot. Bardock, Gokuīs dad, fought Freezer and even then didnīt turn super saiyain, no reason to believe that a Goku without his earthling friends would have made it further.