RottenTomatoes is not an accurate measurement for how older films were received. It can only take into account the vintage reviews that have been archived online, while including reviews that have been written within the past decade. Sorry, but I don't think Cinemablend.com and eFilmcritic.com provide revealing snapshots of how a film was received in 1984.
And that's what we're really talking about here. If we're going to hold KOTCS up against the previous three, we can only do so by putting each of them in perspective.
So then the same holds true for IMDB rankings. Sure, they allow us to see how the films are perceived 20+ years later (at least by a certain demographic), but this is done using votes that were cast from the same modern perspective.
Let's not kid ourselves that either TOD or LC would receive the same nostalgic reaction if released today. Reactions for both were mixed upon their original releases, and today's overly-cynical environment would not prove any kinder.
If anything, all this suggests is that KOTCS will one day be regarded with a lot more nostalgia and fondness than we see today. As we discussed in a separate thread, these films will likely be judged as a complete work- rather than on their individual merits. I say this because it's the way nearly all classic franchises are regarded today. Critics will continue to place ROTLA on a higher pedestal than the rest, but it's unlikely they will make any great distinctions beyond that.