United have major structural problems throughout the club. It is a basket case really. The coaching operation is poor with most players going backwards, they have no continuity in the management team, and the infrastructure is crumbling. In many ways they resemble City in 1981 under Swales.
They still have a huge global fanbase and high revenues but, as a club, they are on the precipice. Even the biggest clubs can go into major decline. Look at AC Milan for example.
The longer the Glazers stay in charge the better as far as I am concerned. That said I believe the Glazers will definitely sell up as soon as they can. They need the money because the rest of their business operation is on its knees. Even if United do get taken over it will take them 10 years to build an infrastructure as good as City or even Chelsea.