So I mostly don't disagree with what you've said however the point which you miss out is that playing the right system can unlock players ability to play to their full potential, so signing the right type of players will make the entire team play better.
Just look at Klopp, he's shown that you don't need world class quality to do well. He's turned a bunch of average players into a brilliant team. If you look at the quality of their squad that team looks nothing better than 6th/7th at best. He's turned average players like Wijnaldum and Mane into world beaters. He's proved that the right system and ideas can make players far greater than the sum of their parts.
That's why when people look at Morata, Matic Perisic they can feel underwhelmed but they miss the real point. These guys can have the same effect Mane had on Liverpool, they can be the missing piece of the puzzle which allows the rest of the team to flourish and I don't just mean allowing Pogba and Herrera to push forward more. When a whole team clicks the effect is much larger than what one great player can bring. Another example is the effect Isco had on Madrid when he came in while Bale was injured. He's not necessarily a better player than Bale but it let them play a different formation and style and they completely dominated an excellent Juventus side in the CL final.
I think these things are not so obvious when you watch games or teams over the season but it explains a lot why managers try so hard to get their man and don't look for alternatives who may actually be better in quality. Look at Klopp being ready to spend 70m on Keita, he knows exactly the players he needs for his system and he'll pay any price to get them. People said he overpaid for Wijnaldum and Mane at the time but we were all proved wrong.
I think if we get the right players in it won't matter that Morata isn't a world class forward or that Perisic isn't a 20 goal a season winger, they will improve our whole team if Mou can slot them into the right system