There are other important things that can have a big impact, besides the ones you mentioned, that is club management. We all know that since Man United was controlled by the Glazers, the club's management has been in disarray, and I think this is the reason why Man United has had a hard time getting back on its feet. Glazer's priority is profit and financial gain over their performance and achievements on the field, therefore, he doesn't really care about the quality of the Club as long as he still makes money.