Bill Weld's invisibility almost certainly helps Scott in a potential primary. Weld has received so little media attention, in addition to being virtually ignored by the Trump campaign, that endorsing him is almost certainly less of an issue for him in a primary than it would be if Weld were polling at, say, 25 percent nationwide.
Agreed, if Trump was facing a credible challenge then Phil might have more to deal with. But again, even Vermont Republicans are still to the left of the mean Republican on some issues.