Barcelona is very lucky to have such an unmatched player, Messi an example of loyalty. As long as you honor your club, your club will honor you. No matter what the circumstances, respect and love for your club must always prevail. Mbappe would like to show his talent elsewhere. Here, the difference is clear, that's his own business. The truth is that not every player will commit to his club until his retirement.
I would not compare the cases if only because the clubs are very different. Barcelona is an institution for decades, the pride of a large region and also a political dimension (read about the role of FC Barcelona in the Franco dictatorship). Therefore, it is understandable that not only Messi remains loyal to the club until the end, but also some players before, such as Pep Guardiola. On the other hand, PSG, a second-rate club that has become big in recent years because sheikhs have invested billions in it to boost their egos. The club is an art project. How can a player commit himself to such a club permanently?