Having enemies makes Z Warriors more motivated to train?

[SIAD]

If within the series there were no enemies, the Z Warriors would have less motivation to train and therefore, they would not be as strong.

 

[SSJ2]

It didn't motivate the humans to train after the 3 year gap

 

[Boo Brand Milk]

If within the series there were no enemies, the Z Warriors would have less motivation to train and therefore, they would not be as strong.

Tien, Piccolo, Goku, Vegeta, etc train no matter what. I think an antagonist let's them know what level of ki they're dealing with.

 

[Boo Brand Milk]

It didn't motivate the humans to train after the 3 year gap

Tien was Fat Buu level

 

[Captain Cadaver]

They obviously wouldn't train as hard without a foe to react to, since a lot of training methods wouldn't even be available to them without encountering enemies (Kaio's training, Gravity training, etc.). However, some of them would still train as hard as possible. Goku trains to surpass his own limits first and foremost, so he'd train as hard as he could regardless of their being a threat or not. Assuming we're referring to a point after they came aboard for this hypothetical, Piccolo and Vegeta would do the same as Piccolo is stated by Toriyama to mainly just train in his spare time and Vegeta trained with the motivation of surpassing Goku and (after the Boo Arc) training to better himself. Tenshinhan also trains to see how far he can push himself even if he can't match the others, so he'd still train as hard as he could.
Kuririn definitely wouldn't train as hard without that incentive though, and Yamcha wouldn't train at all after the Freeza Arc, with Gohan not training at all in general.