Indeed, after being abandoned by the players who brought them to the golden age (one of which was Messi), until now Barcelona is still having difficulty getting back on their feet. However, this is not only because the players have left, but there are other reasons, such as Barcelona's finances which are still unstable at the moment.
Financial condition is an important factor for a team, because if their finances are good then it will make it easier for them to move freely to make the team even stronger.
It's true, Barcelona has slumped in recent seasons not only because several key players, namely Lionel Messi, Neymar & Luis Suarez, have been left behind, but also because the club's fundamental condition isn't good. Currently Barcelona is in 3rd place in the La Liga standings, below Girona who's in 2nd place with a distance of 1 point. Both teams have played the same number of games, do you think Barcelona can be in 2nd place at the end of the season?
I don't know. If think if you lose a whole generation that has dominated world football within 5-6 years, it's hard to see how a club would instantly fully recover from that. Real Madrid has done a better job, it is as simple as that. They have risked more with their transfers and got players like Bellingham and Vinicius Junior on board. Barcelona has some great talents, but when you compare both teams to me it seems that Real Madrid is more grown up despite having so many young players. Barcelona gets Lewandowski and Real Madrid gets a number 5 called Bellingham and the guy scores more goals in a season than Lewandowski does. Overall Lewandowksi has a few more scorer points, but still in my opinion Real Madrid is making the superior decisions.
Now the next transfer is being discussed, Mbappe. This type of player would never choose to go to Barcelona at the moment. But he would have 10 years ago when Barcelona was world class.