Your concern is duly noted. Trump has continuously for the past almost 12 months taken risks deemed by most to be foolhardly and he’s still standing. I don’t see this one as particularly worse than the ones that have before, and the upsides are many.
Trump is not going to debate Bernie saying it seemed inappropriate to debate the “second place finisher”.
Evidently the upsides were not sufficient to warrant the risk. Wisdom prevailed.