Humans learn the Kaioken x20?

[SIAD]

How strong would they be in each Arc and how would the story unfold?

How would it be in both continuities (DBS and DBGT)?

 

[Captain Cadaver]

Considering the humans even at their best were still lagging behind the base Saiyans by a little or a lot from the Namek Arc and onwards, they wouldn't perform any great changes to the plot through their own power. The only possible change would be the extra boost Gero would get from draining Yamcha if the latter has time to go KKx20, which may boost him by enough he can put up a good fight against Piccolo and cause Vegeta to need to intervene, though it's doubtful this would happen in a timeframe where Vegeta could kill him before Trunks arrived and Gero could make his escape. Perhaps Kuririn being able to get the remote from Bulma far faster may change events in the Cell Arc, but I doubt it. The most that would end up being changed on the grand scale of things for Super would be the humans easily beating their opponents in the Moro Arc and be able to assist against Saganbo and Moro, though they'd be able to accomplish little against such threats. For GT, it'd change nothing either when Yamcha and Tenshinhan barely appeared and no hard evidence suggests KKx20 Kuririn would avoid getting killed by #17.

Tl;dr power creep became too much.

 

[SIAD]

Considering the humans even at their best were still lagging behind the base Saiyans by a little or a lot from the Namek Arc and onwards, they wouldn't perform any great changes to the plot through their own power. The only possible change would be the extra boost Gero would get from draining Yamcha if the latter has time to go KKx20, which may boost him by enough he can put up a good fight against Piccolo and cause Vegeta to need to intervene, though it's doubtful this would happen in a timeframe where Vegeta could kill him before Trunks arrived and Gero could make his escape. Perhaps Kuririn being able to get the remote from Bulma far faster may change events in the Cell Arc, but I doubt it. The most that would end up being changed on the grand scale of things for Super would be the humans easily beating their opponents in the Moro Arc and be able to assist against Saganbo and Moro, though they'd be able to accomplish little against such threats. For GT, it'd change nothing either when Yamcha and Tenshinhan barely appeared and no hard evidence suggests KKx20 Kuririn would avoid getting killed by #17.

Tl;dr power creep became too much.

Regarding Krillin from DBGT, remember that he must be at least at the level he was in the Otherworld Fill.