If UA is smart -- and who's to say it will be -- it would offer US elites a status match or at least fast track challenge to maintain their STAR status in the UA/MP program. I expect there would be thousands if not tens of thousands of US elites who will still find STAR more to their liking.

If these UA flights are going to continue to be regular routings and there's no AA alternative, then I suggest you contact UA to see if they will fast track you so the flights taken in the first three months when US is still in STAR won't be wasted should you want to move over.