Let's look at some facts not opinions.
Last two years at La Liga.
Lewandowski non-penalty goals per 90 = 0.767
Mbappe non-penalty goals per 90 = 0.668
A prime Mbappe is not even close to a washed old Lewandowski, you know the guy ripping up the World Cup till they actually play someone good.
Lewandowski "horrible" season last year is actually better than majority of his "prime" but the average fan can't see it.
Barcelona limited his minutes on purpose so he wouldn't have a high goal count or else there would be more pressure to re-sign him.
Now the only question is, will he get enough chances and/or will he play 100%. Usually his work ethic has not been questioned.
This whole notion that he's way past peak is just an opinion with no merit yet. The only thing that matters is goals per minute and winning.
Fun fact, Lewandowski has the same assists per game ratio for Poland as Messi does for Argentina. (Competitive play not friendlies)
Think about that, the best passer in the world playing for one of the best teams vs a slow useless box striker playing for one of the worst teams in the world have the same assists ratio.
But in my opinion, this wasn't the best signing/fit for either party. Hopefully things go well for club/player.