I think Sir Alex was also part of the problems that befell Manchester United when he resigned. News then had it that he was the man that handpicked his successor which was David Moyes. That was the genesis of Manchester United problems, an inexperienced coach handling a team like Manchester United at that time, this was a title winning team that was handed over to him.
Fergie gave a very old squad as his legacy when he resigned. It makes MU struggled for years to regenerated its player. Things worsen when MU could not pick a good coach that could repeat Fergie's history in MU. The club has been downgrading so much since then.
I'd not blame Fergie for that because MU's management totally responsible for that. Also their terrible owners who drained club's cash every season made it worsen.