Besides, they can just duel with "no Numbers allowed" restriction, and then it won't matter if Yuma lose.
It has been established that no Numbers on the line = no Astral dying.
On one of the early episodes, Yuma is shown to have yet another loss against his friend after having 39. And Astral is not shown to be dying.
Another example, Astral isn't shown to be dying when Yuma goes down to 100 LP in his Duel against Shark in episode 84, despite Shark supposed to be holding 32 at that point, since he used it on episode 78.