Despite his age, Lewandowski still makes a huge impact for Barça. He has a huge impact in every match he plays for Barcelona. If he were 10 years younger, Barça would be the same as they were during that unbeaten period. He's a truly exceptional striker. We know that his return to the team would clearly be a huge boost for Barça. Every player would want to work with someone as experienced as him.
Lewandowski can contribute a lot for Barcelona even he is very old as he is talented, skillful, experienced and very professional but no human and player can win against time and ageing effects forever. Barcelona can enjoy amazing contributions from Lewandowski until he hits his physical limits and has declining performances in both appearances and quality.
With a very old player like Lewandowski, that declining performances can appear anytime and can not be reverted, so Barcelona must have different plans and players as replacements on Lewandowski. If they struggle to replace him in one, two or three seasons, their performances and results can be affected a lot, then finance will be considerably affected negatively too.