yes the stress caused by the national championship or other challenges certainly tires the team which will then be much more approachable by the other teams that will challenge it, it is certainly difficult to play all the competitions in a row, but it is like this and it is necessary for the big clubs to have two teams to compete at high levels in all the competitions
More precisely not two teams, but reserve players who are as good as the main players in the club so that the club does not experience a sharp decline in strength when there are several main players who are injured due to playing optimally in all competitions including in difficult competitions such as the Champions League. However, the problem for some clubs is that having reserve players who are as good as the main players is not easy because players who have good talent will certainly be very reluctant to sit on the bench.
It only makes sense when those reserved players are alternated more often so that they keep their forms and confidence. In a case where a team has reserved players that are as good as the starting XI or almost as good as them and rarely use them they begin to lose form and when the coach decides to use them because they end up underperforming. Some players get insured because they have been over used whereas they had substitutes whom the coach would have given a chance to prove their skill while the other maintains fitness.