IIRC, the issues this summer were more directly related to UA not timing the completion of Polaris retrofits on the 764 in time, resulting in then being short airframes that could operate the route. That said, I’m not sure what would prevent UA from operating a 772 on the route, which they did (and I flew) in the winter months. I know Hawaii is a little less popular of a destination for Americans during the summer (given there are lots of other warm-weather options), but I would think there is enough demand to sustain at least 3x weekly nonstop during the summer, whether on a 764 or some other widebody.