His red trunks breaks the color. It brings balance to the color of the suit.
And there is the old one. He appears like a wrestler with the trunks. A strong man. Don't wrestlers wear undies when they enter the ring? Professional wrestling is pretty popular. If i see Goku for the first time i can recognize him as someone related to martial arts. Kung-Fu, Karate or something. One associates such costume to martial arts. Undies are worn by muscleman. Maybe the earlier circus strongman on which it was modeled or even now when wrestlers are associated with the undies.
His muscles bulge everywhere. He wears tights. Look at his abs. His costume is skin tight everywhere. Why does it not bulge there? It is illogical.
There is so much thought put into his costume. Think about it. Why does he have a yellow 'S' at the back of his cape? It is because even if we see him from the back we can recognize he is Superman. Why does he have a spit curl? That is an 'S'. Even with a close up of his face you can recognize Superman.
You can see the 'S' in this picture. You know he is Superman.
Can you see the 'S' on his chest? No. But 'S' is on his head.
When artists and creators make something they put a lot of thought behind it. How will the character pop up on the page? How can you get great shots from every angle? How to balance colors? How to make the character eye catching?
Removing his undies to fix him is like solving a problem which never existed in the first place. (Maybe it was the work of the blue nude god Dr. Manhattan? He does not wear undies. Or nothing for that matter).
Is lawsuit a problem even now? I thought it was over.