Selling players who no longer have high productivity is one way for clubs to earn money and buy younger players to slowly build a strong performance foundation for the club, but all this cannot be done haphazardly because if you think about selling players who are no longer productive, of course very easy but there are also some negative impacts that must be faced.
For example, Robert Lewandowski, who is too old and actually doesn't have high productivity anymore, will definitely have a bad impact on Barca's attack line, which will experience a decline in performance when they don't get suitable replacement players.
Actually there are many other ways but it all depends on how management handles all this because currently Barca has big financial problems which really need to be resolved slowly.
Selling old players gradually may be the best way.
Of course, Xavi and management are not ready to let go of Lewandokwsi in the transfer market because Lewandowski role is still needed by the team. Another reason is that Barcelona has not found a player who is ready to contribute goals like Lewandowski has done. But it is true that he is getting aging and in my opinion Barcelona will use his services for the next one season because Lewandowksi is getting agin and his energy is starting to disappear when he has to deal with younger players. Yes, of course they can sell older players to buy young, better quality players, but Xavi probably won't do that.
Barcelona certainly has to look for quality strikers to be able to compete strongly next season. Lamine Yamal is starting to show his best qualities but he is still very young and of course this is where Lewandowski is needed to support Lamine Yamal performance. Apart from that, it is difficult for Barcelona to buy expensive, quality players because their finances are still not improving, so this will prevent them from hunting for strikers who perform well in the transfer window.