The reason behind Valentina Allegra de Fontaine's acquisition of the Avengers Tower before Thunderbolts* and its implications for the MCU are hinted to by director Jake Schreier. Following The Avengers' Battle of New York, Tony Stark renovated his Stark Tower to serve as the official headquarters of Earth's Mightiest Heroes. The titular team lived in the Avengers Tower for a number of years before Stark sold the structure to an unidentified bidder. Valentina Allegra de Fontaine purchased the Avengers Tower and converted it into her own headquarters while the Avengers continued to operate out of the Avengers headquarters in Upstate New York.
 

Director Jake Schreier discusses the ownership transition of the former Avengers Tower in an interview with Empire, Thunderbolts*.  The purchase of Avengers skyscraper by Val is a bad sign for the world, according to Schreier, who calls Val "the person you would least like to own that tower."  Schreier implies that things will take "a darker turn" going forward, although he doesn't specify Val's precise motivations for the purchase.  See Jake Schreier's complete remarks below: “We know what that tower means. Who’s the person you would least like to own that tower, that means so much to so many people? Can you use that as a symbol of things taking a darker turn?”

Though Val's plot to put together their own Avengers counterfeit squad is just now coming to fruition, Valentina Allegra de Fontaine may have bought the Avengers Tower from Tony Stark prior to the Avengers disbanding after Thanos' defeat in Avengers: Endgame.  Even worse, given that Sentry will go on a rampage throughout New York after hurling red Guardian over the Thunderbolts Tower, Val's preparations will quickly go horribly wrong—or right, depending on her true motivations.  Given that President Ross just demolished the white house in his role as the red Hulk and Wilson Fisk is the new mayor of New York, Sentry's attack is simply another indication of a gloomy future.
 
 

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