Football is cyclical of course, you can't dominate forever. Your academy will dry up, you'll make signings that don't work out, perhaps you'll make bad managerial appointments.
But you also have to accept that a club with among the most resources in the world cannot accept such a long period of mediocrity. Its not isolated decisions, you can make a compelling case that we've made 4 bad managerial appointments in a row. We've had no consistent philosophy of how we want to play (Moyes was get it wide and cross, LVG was just keep the ball forever, Mourinho was standard Mourinho and Ole didn't know what a philosophy was). We've spent a lot of money on players that haven't worked out for whatever reason (ability, mentality, coaching). We've engaged in vanity signings and neglected the balance of the squad entirely.
At some point there's no point criticising managers, coaches and players every week because the club are responsible for appointments at the top that feed down. The club rots from the head down. We have no real football expertise in the club and no-one who sets the foundation for what we are trying to achieve and how we will achieve it.
Many of you will know from my other posts that my wife and boys support the MK Dons and so I've gotten roped in because its a hell of a lot easier, and frankly more enjoyable, to follow them. The difference between how United and the Dons are run is eye opening. The Dons football staff are excellent. Every single season we will find players in the lower leagues and within 12-18 months they've been moved on for profits because we aren't big enough to retain top talent (e.g. we just sold Matt O'Reilly to Celtic and he's already a starter and getting rave reviews, Rhys Healey went to Toulouse where he's tearing it up and West Ham are now after him). However, the fans never really begrudge this because we know we need to do this to survive and we also know we have the structures and people in place to replace these players and still take us forward. We have a philosophy of playing that survives managers, a philosophy of what we want to be as a club and the type of players we want to sign.
United, compared to this, are an absolute basket case, its embarrassing for the fans and it should be embarrassing for the board who have consistently failed for a decade.