Manchester United are not going through a good period, but let's say that calling them mediocre is not an insult but an observation
i am sorry, i remember the good times when they were the most feared team in Europe
they have a team worth 694 million euros but they are not performing as they should in the Premier League, i would ask myself two questions

It was Manchester United past history. Thy're nothing now. Look at the comparison between their past performances compared to the their performance in the modern football. There is a huge gap, yeah. United's 12 years total game lost is now higher than their lost when fergie was there for 26 years.
They looked so desperate to found the next fergie, but believe me, there will be no fergie's copy paste. They will never find someone who can suit them well like him in modern football. Not until they also changed their owner.
Calling them as a mediocre club is not only an observation, but it based on the reality showed on the table.
All fans miss the man who gave them glory back then, but as you said it is very difficult to get nowadays and have to accept the fact that it is really difficult.
I have never seen Manchester United perform as great as I have seen them perform in the past. And where Old Trafford was so scary back then, now it is a place full of jokes and even a place where other teams use it to win.
The decline of a team is a natural thing, they can still rise even though it is difficult, who knows how long it will take them to find their glory days like before.