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.