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.
In a few months he will be 36 years old, of course the club will start to doubt that his performance will remain the same but if the Barcelona midfielder can provide good passes he is still likely to score many goals for Barcelona, but what you say is true because he is a typical player who has played since early minutes then it will be quite difficult for him to remain productive when the Barcelona coach starts using him only as a backup striker if Barcelona can get a new striker and the player can adapt quickly. However, financial difficulties could be a big obstacle for Barcelona getting a very good quality striker so Lewandowski is very likely to remain the main choice and next season he could start looking at the possibility of moving to another league where the competition is not as fierce as Laliga.