You say this as if it was universally valid and any player could be replaced, but this is definitely wrong. There is a class of players you can't replace. How would a club replace Messi with a player of equal quality? Or how would a club replace Mbappe or Ronaldo during his prime? For most players I think you are right, but is always a hand full of players that are above everyone else. They produce numbers and chances like nobody else.
It is the same for extremely good goalkeepers. If they get injured, many of the teams have instantly a problem. So obviously it does matter how good a player is when it comes to replacing them after they retire or leave the club or are injured.
I said it and I stand by my words. Every player is replaceable. Except for only two players, Cristiano Ronaldo and Lionel Messi in their prime. Except for these two every player is replaceable, at least so far. Even Mbappé is replaceable.
Do you remember the magical words of Pep Guardiola? When he said "
we cannot replace him, we just cannot" about Aguero?
In the end, he was probably the easiest to replace. So I never believe that no player is replaceable. Only the two that I mentioned were irreplaceable in their prime and those two are exceptions. And remember exceptions do not make the rule. Maybe a few very select players are not replaceable at a certain time. But in general, it is not true that you cannot replace a player. With time and money anything and any player is replaceable.
I 100% agree with you on Aguero and don't forget that Pep Guardiola uses these praises as part of his communicative strategy. He says about everyone of his players that he loves them even when they are sitting on the bench. He calls literally every player irreplaceable, it is just Guardiola
See, I wasn't wrong when I brought up Messi and Ronaldo in their prime and I think that Mbappe has some elements in his game that are almost non-existent twice in the world right now. Replaceability means availability right now in the present. Sure there might a player in the future who could perfectly replace Mbappe, but I don't see that player today. Vinicius Junior has his own set of qualities as a left winger.
But let me put this differently again: I am not saying that no other player could play there without the team losing quality in their game. When Vinicius Junior was injured, they did a good job in "replacing" him as it is still Real Madrid. But you can see how nuances changed in their game when Vinicius Junior was back and reached his top level again. It's a difference. The same happens when Mbappe is injured and PSG has to get along without him. Of course they can put another high quality player there, but in my opinion there is no doubt that you can see the difference when he is missing.
And availability really is a problem when a club doesn't have time to plan. It's not like they can just pick any player in the world and boom problem solved. A lot of things have to work out in terms of timing and stuff.
I get your point, but I would still say that for some players it is close to impossible to make up for their absence under most (if not all, see Messi or Ronaldo) circumstances. There could be another Ronaldo in 15 years from now or another Messi, but that doesn't help the Madrid or Barcelona of today.