Everybody knows that R. Lewandowski has performed excellently for Barcelona, but still it won't be a bad idea to sell him if they find his replacement.
Lewandowski is very talented, experienced, professional and also highly ambitious even at his very old age so that he has been able to prolong his career and play at very good level. He is old and has more and bigger burden of ageing season by season so Barcelona must seriously consider possible replacements on Lewandowski in near future.
Julian Alvarez, in my opinion, will do the job well. R. Lewandowski performance this season shows that age has begun to affect his football ability because with the chances that were created for him this season, I don't expect any goal competition between him and Kylian Mbappe because Mbappe is always struggling creating chances for himself while other players are creating chances for Lewandowski. They need active striker like Alvarez it they want to keep to their performance.
Julian Alavarez already shown his quality in Manchester City, Argentina national team and Atletico Madrid, so he has proven both his talent, and ability to play well against expectation and pressure. He will be a good replacement for Lewadowski in Barcelona but chance of having him from Atletico Madrid is not too high. Barcelona have problems in finance and they won't be able to spend too much money for purchasing Julian Alvarez while it is like only reason for Atletico Madrid to sell Alvarez. With an important player like this, Atletico Madrid won't sell him for cheap transfer cost.