If Lewandowski stays and Barcelona finds another striker, he might be facing a rough next season when it happens again that he can't deliver for several games in a row. This season it was still no problem for him as he didn't really have too much competition on his position, but his time may have come now and Xavi takes the consequences and lets him sit on the bench. The good times for him come to an end now I feel. He will either be scoring a hell lot of goals or he will get less play time, which could quickly lead him into a negative spiral. I think he is not a player who is very good at coming from the bench and being 100% ready to score the next moment. He needs the whole game to show his best. I am sure he is going to closely monitor what his club is doing in terms of transfers.
The problem is that Barcelona does not have sufficient budget to bring in new players unless Barcelona releases some expensive players to cover it. Apart from that, Lewandowski's salary may be a consideration next season I'm not sure Barcelona keep him while they're overburdened and Lewandowski's contract will expire in 2026 so there is a possibility that Lewandowski will be released to the Saudi Pro League. Currently, Lewandowski is still active in scoring goals but like you said because there are no competitors there different from the players at Real Madrid who have several great players.