User blog comment:IamJakuhoRaikoben/Chapter 463: Black Carpet, Review./@comment-7418318-20151123101022

They didn't really have a choice in "trusting it to Natsu", because in this case that's just pretty-talking for "there's nothing we can do to help him anyway, so let's hope he'll pull through".

I mean even if they rushed to Natsu's help, what would it accomplish?

Against those numbers AND Zeref, they wouldn't make much of a difference, but at the same time they would expose the other flanks, that they were about to strengthen.

Better to sick with the original plan and hope that Natsu can hold out long enough for them to move on Zeref with their combined forces (or is captured instead of killed).