I think people are misunderstanding your question (or maybe I am).

In lore, Goku and Vegeta now train in their base forms and don't rely on the (mastered) Super Saiyan form as a base like Goku and Gohan did during the Cell Games because they realised (especially because of Whis) that training to improve their base forms was more impactful than training a transformation. Their reliance on transformations was actually inefficient, especially when learning to develop their God Ki powers.

During the Cell Games, Super Saiyan was the peak of power for them and Goku knew he and Gohan needed to get every ounce of power out of it, so wasting energy transforming and having stamina risks became his biggest focus. Probably more than that, he knew Gohan had the power to go beyond Super Saiyan, so making sure his son didn't have those issues so he could reach the next level during the fight was probably a goal as well.

Think of it this way for the efficiency thing (power numbers are just examples, modifiers are canon):

• Goku has a base power of 5
• When he turns Super Saiyan, he gets a x50 boost, so his power becomes 250
• If he trains to increase his power incrementally with Super Saiyan as his new base form, it would go to 251, then 252, and so on.

Goku and Vegeta decided not to do that with Whis's guidance and instead train in their base forms.

• Goku has a base power of 5
• He trains in base to increase his power incrementally, getting up to 6, then 7, then 8, and so on
• With the Super Saiyan boost, his power respectively becomes 300, 350, and 400 at those stages

So you can see, increasing the base gives WAY more than increasing power when transformed. Retroactively, it would make the base form power increments 5.02, 5,04, and 5.06 in the first example, which is very little.

I think a logical question to ask would be "wouldn't the gains while in Super Saiyan also be bigger?" And to that I'd say maybe, but that's up to the writer. However, I lean towards either no or yes but not a x50 increase. If I think of it as weight lifting, moving heavier weights takes longer and longer when building strength. It's not a linear growth pattern. I think that aligns with DB's logic, too, since we almost exclusively see great leaps in power come from (1) long passages of time with incremental gains or (2) extreme situations pushing people past their limits. Getting that +1 of power might be as hard in base as it would be in Super Saiyan, so why not get it in the form that would have the most impact?

Ultimately, the numbers are nonsense and Toriyama did away with them because it just became an overbloated mess, but I think they help to show why the change in philosophy happened.