We've been infected with cancer ever since 2005, when the Premier League allowed the Glazers to buy the club through a leveraged buyout, which is now illegal by the way because it's an insane arrangement that would sink any profitable institution.
Sir Alex was still able to keep us successful for another 7–8 years with 2 hands tied behind his back (bringing in Michael Owen and Antonio Valencia to replace Tevez and Ronaldo LOL, while players like Ribéry, Hazard, Benzema, and Aguero were available) because he's the GOAT
And that's how bad things already were: fans were organizing Glazers Out protests as far back as 2007–2009, when we were winning three Premier League titles in a row and reaching back-to-back Champions League finals. Imagine how bad the situation must have been for fans to be protesting even while we were dominating English football.
Once Sir Alex left, nobody was ever going to have a chance at success under the same ownership. The final nail in the coffin was Sheikh Jassim withdrawing his bid on October 14, 2023. That was the day Manchester United truly died forever. We might win a domestic cup every 3–4 years, but we're essentially no different now from Everton or Aston Villa.