It is clear that it is impossible to change the past and we must look forward. But I think you will agree that Barcelona is close to being titleless this season and it will be a very unusual situation for Lewandowski as he is used to getting at least the German championship every season + scoring a lot of Champions League goals until the late stages tournament. Given Lewandowski's age, I don't think he won't regret that one of the last years of his great career went like this.
Lewandowski's decision to prefer to leave Bayern Munich must have been thought out carefully and thoughtfully. So I didn't think that he would regret his decision, moreover there was no point in regretting it because he couldn't change his decision. Yes maybe there will be something missing for him, but it doesn't matter in my opinion because he also feels the competition is more challenging than the Bundesliga.
Bayern Munich and Barcelona are two differen team with different work load. When Lewandowski was in Bayern Munich I don't think the pressure he was facing there can be compared to what is in Barcelona. Bayern Munich used to be the strongest team in the Bundesliga league, and with no tough competitors. If Lewandowski is not doing more in Barcelona I won't blame him because Barcelona is competing with more tough teams than Bayern Munich.