Would you swap it all for your perfect ending?

[PsyLiam]

It wasn't so much that he became obsessed with Saiya-jin's that he'd inadvertantly written out the Humans. He'd not planned on the rest of the series when he made Goku and Freeza so powerful, and there was just no way for the Humans to catch up to the Saiya-jins. He really didn't have much choice.

That tendency had already begun with the Saiyan saga, when Goku and ONLY GOKU had a chance against Nappa, let alone Vegeta.

And he could have given the humans some form of power-up. But he didn't. The meanie.

[Xyex]

He gave Krillin the Guru Power-up.

But seriously though, how would he have worked that in? Have Bulma turn them all into Cyborgs using the plans for 17 and 18? Or maybe have her inject them with Saiya-jin DNA making them all half Saiya-jin's?

You do have a point about Nappa and Vegeta though. If Toriyama-sama hadn't come up with the Kaio Ken technique he could have lowered the powers of Nappa and Vegeta to a level where Piccolo and the others could have at least beaten Nappa. Of course, this means that all the other villains would have needed to have been weaker too.

[Dayspring]

Humans were "written out" since the 23rd budoukai. Krillin and Tenshinhan got they're asses kicked in the semi-finals, and these two are by far the strongest human fighters on Earth (with Yamcha and Chaozu not too, too far behind). Tenshinah even acknowledges after he lost that he's far from Goku's level. Then in the Saiyan saga we find out that the humans are among the weakest species in the universe, and Saiyans the strongest. Now make Goku+Vegeta the strongest Saiyans of all time, and shove them in the top 3 strongest in the universe. It's not surprising that they were focused upon in the latter sagas, as part of being a Saiyan means you have an infinite potential and that part of being a human means you have a very limited one.

[PsyLiam]

I don't think it would have been too ludicrous to invent some sort of power-up for the humans. After all, we accepted that a giant cat-fish person can give Goku an awesome power-up technique within the space of a few panels. We can accept that people who are only 1/10000000th Saiyan can still do the blonde hair RARRRGGGHHH thing. Why can't some random being with amazing powers have somehow unlocked a "Super Human" potential thing?

But as I said, it wouldn't have mattered. If Toriyama hadn't have invented the Kaio-ken technique, everyone else would have still had their arses handed to them by Vegeta (although story-wise, you could have had Piccolo beating Nappa). As Dayspring says, by the 23rd Budokai the formula was pretty much set.

1/ Mild comedy is ensuing, until OMG ULTRA POWERFUL BAD GUY appears.
2/ Goku doesn't fight him for some reason.
3/ Everyone else does. Sometimes all together, sometimes in gradually increasing order of strength.
4/ Kuririn will lie on the floor, wishing he could help, but not actually doing anything. Sometimes Yamcha will do the same, but upright and from further away.
5/ If Piccolo (pre-Buu saga) or Vegeta (Freezer to end) is present, they will usually tie with the OMG POWERFUL BAD GUY. This will cause one of two things to happen:
Option a: OMG ULTRA POWERFUL BAD GUY will transform into OMG SUPER AWESOME ULTRA POWERFUL BAD GUY.
Option b: OMG ULTRA POWERFUL BAD BUYbe replaced by OMG SUPER AWESOME ULTRA POWERFUL BAD GUY.
6/ OMG SUPER AWESOME ULTRA POWERFUL BAD GUY will beat up anyone still alive.
7/ Kuririn will continue to lie there.
8/ Goku will come along, beat up OMG SUPER AWESOME ULTRA POWERFUL BAD GUY, and save the universe. Usually with the aid of a New Secret Technique.

(The Cell saga is the exception to this, where in rule 7 "Goku" gets replaced by "Gohan". DIFFERENCE!)

So it doesn't really matter what power-ups they gave to everyone else. They'd still lose.