I think it is not the difference in motivation that matters (it is approximately the same for everyone), but in the paths of the national teams. Someone will reach the final and be more tired, while someone will leave the tournament after the group stage (or will not take part in the World Cup at all like Italian players) and have time to rest. Someone will get injured and miss time while being treated. These are random factors that can hit top clubs (most of the players in which will participate in the World Cup) hard.
However, the clubs will have to take this on the chin and move forward from here. Because it is obvious that they cannot do anything about that. There is a very real possibility that a player is so excited about the World Cup that he may injure himself while training or playing for the team in the World Cup, because he is so excited about the World Cup.
As a matter of fact, some of these injuries can be downright pathetic. Injuries can obviously have a detrimental effect on how well a player is going to perform for his respective club and that also brings in doubt about if he will even be able to perform at all because of injuries. It is something that the clubs will have to endure because there is no way around it because it is a necessity.
I don't think clubs will tolerate this shit forever. Clubs do business, officials dispose of cash flows to which they should not be related - this is a crooked scheme and sooner or later it will be corrected. The idea of the Superleague appeared in response to this endless nonsense from UEFA and FIFA. If you notice this does not suit very many, not even businessmen, but also athletes - for example, Klopp criticizes the calendar from the beginning of the season, which turned out because of the World Cup and other crappy tournaments like the League of Nations.
