My brother looks at things from a statistical perspective. He explains our decline as the 'reversion to mean' statistical theory.
Basically you have above average stand out occurrences, ie Mo has a way above average player (positional sense, dribble/shoot/score ratio, or VVD wins above average one on one or Robbo/Trent have above average assists). These abnormal statistical performances are not linear. They can't and they don't happen in every situation, ie every game. Mo can only play brilliantly in 15 games per season, VVD 25 and TAA in 18 depending on position, physical demands, age etc..
The more you play a stand out performer, the more their brilliant performances are dilluted by average performances;and should the team be over played, the less dominating we are and the greater the chance we'll lose. Slobo is a great example: started off strong then reverted to mean, a shadow of his early season performance.
In short, the more you play a great player, the less-great they play with the outcome they revert to 'mean'. Hence we started out going for 4 cups and we ended up with the Carabao. Only because it was the first cup to be finalised. Nunez isn't a great talent so over playing him has actually worsened his effectiveness not improved it. His shoot/miss rate now is worse than it was at the beginning of the season. Klopp has no idea about reversion to mean, he somehow believes if he's over played he'll improve.
This is why City with their bigger spend on a greater number of quality players maintain their momentum because Pep can rotate and keep his key players fresh ensuring their performances remain 'above average'. The greater the overall quality of the squad, the greater the ability to maintain momentum, ensure your over-achievers remain fresh and neither individual players nor team revert to mean.
Of course you can get this wrong by spending tons of money on the wrong players such as Chelsea have done, or by having the wrong manager as Man United repeatedly do.
Mo, VVD, TAA, Robbo and Ali have been overplayed since 2019. Elliot, Jones, Gakpo and Nunez aren't good enough. MacAllister is a solid addition and Slobo will be if he is used intelligently. The new manager shouldn't despair, if we focus on just CL and EPL next season and bring in a couple of core players then we can go again, but we have to have a plan that sees our first team core go from 14 players to 20, to 24 over the next 3-4 windows. Chelsea and right, you can't avoid spending the money.
Google reversion to mean and read up on it.