Much as there's plenty of validity in pointing out that sacking Mourinho isn't going to automatically fix things and that there are much deeper issues within the club, it shouldn't really be used as an argument for keeping him on either.
The fact we've been consistently poor since Fergie left in spite of having several managers isn't an indication that the manager's themselves aren't the problem: it's just an indication that we've either appointed poor managers, or appointed managers who've done poorly. Moyes was always bound to be a dud in retrospect, LVG was too stubborn, and Mourinho's arguably quite outdated.
Yes, we shouldn't just sack a manager every time we enter a poor run of form, but by the same token it's silly to persist with someone just because he's another failure after a line of failures. When we had a string of shite goalkeepers in the 2000s we didn't concoct crazy arguments that the reason for those goalkeepers failing was to do with anything other than the fact that most of them just weren't particularly good: we went out and signed a top-class goalkeeper who fixed our problems in that respective area. Hypothetically, as an example, if Lukaku's shite this season, and we replace him with another striker who also happens to end up doing poorly for us, we'll probably look at replacing him again instead of accepting the fact that we consistently keep bringing in players who aren't up to the task in a certain position. A similar principle should surely apply to any manager we have.