On the one hand, I’m ready to believe that Lewandowski’s motivation decreased because once he got to Barcelona, he clearly felt that this was not a transition up, but definitely a downward one (Lewandowski became very familiar with the Europa League and lost the habit of playing in the Champions League playoffs while playing in Barcelona haha), but after all, the change of situation should have had a positive effect on him. Most likely it’s a matter of age - he is actually 35.5 years old and this is the age when most football players have already finished playing football. Miracles rarely happen (Ronaldo), so it should not be surprising that Lewandowski, having worse physical capabilities, also has worse motivation.
It is a matter of age and less a matter of motivation. The reason I am saying this is two-fold: on the one hand he declined offers from Saudi Arabia and I think someone who is undergoing declining motivation may get to a point where the desire for more money outweighs the passion for playing the hardest games possible. On the other hand I watched a few interviews he gave in German when he explained why he wanted to go to Barcelona and leave Bayern after breaking and setting all these records. He said that Barcelona has been a lifelong dream for him and he felt it was the perfect time to give his career a last challenging shot. He was super happy about his little children and his wife being able to live in a wonderful, sunny place and he was on fire when he was speaking about his new challenge in Barcelona. It didn't really come across as if he was taking that step to slowdown. I rather thought he wants to prove one more time that he can do it at another top club at his age.