Years ago all economy passengers were equal in assigning seats, although when I first became a frequent flyer United would block the seat next to me as an elite so that if it was possible to have an empty seat next to me I would.
And then they introduced economy plus seating, extra legroom seats that elites could reserve free (and at the time it was also available to full fare customers and to Star Alliance Gold members too). United didn’t initially monetize those seats.