OK so there is a lot to unpick and unpack here.
As a United fan, since the departure of Sir Alex Ferguson, I would have expected United to be competing for and winning trophies. Even to the extent that, that success would not have been at the level of the Ferguson years, I would not have expected the level of drop off to be where it is. Not only that but I do not think that at the time of Sir Alex's departure there is a single United fan that would have expected the club to have gone nearly a decade without a PL title and four years without any trophy whatsoever. Do you?
That said, you make a good point about the coaching staff because we have made a number of poor appointments due to negligent due diligence like Moyes and Ole. But even here, we recruited two managers in Louis Van Gaal and Jose Mourinho who were proven winners of multiple league titles and Champions Leagues. Even if the expectations of Moyes and Ole were lower, would it have been unreasonable to expect Van Gaal and Jose to meet the reasonable expectations of success?
However, where I disagree with you strongly is with regard to player value. In my opinion I cannot accept that the current United squad is not good enough to be competing for trophies at the highest level. For me that is an easy out for players who are playing well below their potential. It was not that long ago that United fans were comparing Maguire to Van Dijk, that Luke Shaw was being described as the one of the best left backs in the country, that Bruno was being eulogised as one of the best midfielders in Europe, that Jesse Lingard had revived his England career following his loan spell at West Ham. I could go on and on. Even with inflated transfer fees, the failures here are not due to player quality but rather consistency, coaching and culture.
Taking the earlier comparison with Liverpool, how is it that with the exception of Van Dijk and Allison, Klopp has managed to take a clutch of c£40m players and turn them into PL and CL winners? Salah was a Chelsea reject, Andy Robertson was a no name from relegated Hull, Diogo Jota was a journeyman pro from Wolves. The argument that the players at United's disposal have not been good enough to be more successful, when the likes of Klopp has achieved more on a lower
gross and net spend simply does not wash. Lest we forget in the time since we last won the PL title, Leicester managed to achieve that accolade on a shoestring.
On a final note, you rightly point out that both clubs have won three trophies since 2013/14. However, to be honest whilst they are the same in terms of quantity, there is a false equivalency in terms of quality. The Europa League and the Champions League and the FA Cup and the Premier League title are not the same. Yes, success is absolutely relative.