A great showdown might not be great business. Yes, Disney and Universal are competing with each other, and because of that they have been stepping up their game over and over again. Your scenario would be akin to a Game of Thrones finale being aired during the superbowl. Companies plan these launches specifically to avoid competition. When new blockbuster movie comes out, competing studios will delay their blockbusters release.
Universal could surely open a new gate during 2021, but they have no financial interest in doing so. The only possible interest they would have would be to take Disney's revenue down a notch during a period of expected highs, even at their own expense, and I don't think Comcast is so petty.