Absolutely, that's what I am thinking, too. Ten Hag's success will depend on how quickly he can rebuild the team. With the current squad, you can't play at the top in the long term. Man United will definitely have to invest a lot of money. It's main problem in recent years has not been the coaches, but the players. But I fear that the squad can't be rebuilt fast enough for the new coach to succeed.
Clubs (and particularly big clubs) can be far too impatient, they bring in a big name coach and expect instant results. Of course, that's unrealistic. First he needs to familiarise himself with the entire squad, to work out what he needs in the transfer market. Then United have to secure those players - which may be tougher for them than usual, as they don't have the prospect of Champions League football to offer to potential new recruits. Then, once they have the new players in, it takes a lot of time to get them to fit together with the current squad into a cohesive unit. And finally everyone - current players as well as new ones - have to get used to the new manager and his tactical style. Even if you bring in the best players in the world, it's almost impossible to guarantee instant results. They do indeed need to give him a few years, but you'd expect that what will happen, if they fail to qualify for the champions league again, is that they'll be looking around for another new manager this time next year.