United have been badly run for years, the Glazers bought the club on the back of loans that were subsequently dumped back on the club after United were previously run on a debt free basis. They hardly put any of their original money into the purchase, but choose to take a large dividend out every season, the sooner they are gone the better.
OGS was never good enough to manage United, and only got the job permanently because of the run they went on when he first took charge, but that might have been because of the freedom the players had after Mourinho's tactics. The only position they are seriously lacking in is defensive midfield, but the rest are badly led with no structure or tactics, so no wonder they are a shambles.
As for Ronaldo, he is supremely fit and I can see him still playing at 40 years of age, but he will need runners around him as you can see now that he reserves his energy for going forward, you won't see him being part of a high press.