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The rankings aren’t real. Never have been. There was no magical point in history where the rankings reflected the “truth”. They are and always have been a marketing tactic to sell subscriptions.
So to answer OP’s question, nothing happened to those colleges. They were never T50 because there is no such thing as the “top 50 universities”. They may have improved in some ways, stagnated in others, and even regressed in some others. Just like every other institution on the planet.
This 1 million percent. Do your own research on the school that is right for your kid. Start with the Fiske guide. US News is garbage and any list of "the best" as told by a magazine are mostly for marketing. If you look at their metrics you will see that.
Both kids in HS have it off - one public and one private. I own a business and do not give it as a holiday.