I get the theory, but the numbers don't back it up. The season before Mbappe moved from Monaco, PSG scored 83 goals. His first season with PSG they score 108 goals. Then 105 goals. The Covid season was curtailed but on track to score 96 goals. Then 86, 90, 89 and 81 (but with 34 games). He leaves to join Real and it goes up to 92 goals - so an improvement but fairly average compared to Mbappe's PSG career. And this season PSG are projected to reach 75 goals, their lowest in 15 years.
For Real the figures are similar. We need to go back a few years to when Real had a stronger team and a stronger midfield to reach the point when they scored a lot more than they typically score nowadays. That is the main point I would make is that we are judging the individuals by the outcome of their teams, which isn't right for me.