The recent hotels use buses though, not the boats. I think Endless Summer is a good bridge between the two resorts. The guests staying there will likely see little difference in what parks they go to, because they'll be on hopping on a bus either way.
In any case, I can see Universal only opening 1 new resort, a flagship one close to the new park, first. Then have more come online in the following decade. I do not see any reason that everything must open at once.
But I also see them continuing the trend of opening a new hotel (or expansion to an existing hotel) once a year for the foreseeable future, until they reach whatever new internal goal of total rooms they hope to reach.