Lewandowski is a prolific striker at Barcelona as well. However we haven't been able to see enough level of contribution by him in big matches of the Champions League. It was like he was a more different striker than his La Liga performance. He wasn't having issues against weaker opponents in the Champions League. But he was a failure when it comes to facing teams like Bayern Munich in the group stage. He will need to concentrate more on that kind of matches from now on as those ones are critical for making it to beyond the group stage.
Your thinking makes me feel that Lewandowski is carrying Barcelona on his shoulders and is fully responsible for any result Barcelona get as a team. He is a single player and even a prolific striker, he does not represent Barcelona as a team. A player is not a team and if Barcelona want to win big clubs in Champions League, in Europe, they must win it as a team. They must become a stronger team to achieve it and Lewandowski can not help them only by himself.
I also think that Ilkay Gündogan is a great transfer of FC Barcelona. He does not cost any transfer fee and i am also quite sure that his salary demands are very reasonable in comparison to those of Lionel Messi for example and everyone knows that Barca is in a very bad situation financially.
Also i think we should not underestimate Gündogan as a player. He was the captain of the best football team in the world in the last few years. This alone teils a lot.
It is unfair to say Gundogan joined Barcelona because he requires less salary than Messi. We don't know what are conversations between Messi and Barcelona and why they did not sign with Messi as a free player. From Messi words, he said Barcelona never officially contacted him to bring him back to Barcelona. With bad history of Barcelona and their treatment style on their players, I believe in Messi.
Gundogan can be a good midfielder for Barcelona to replace Busquet because he is familiar with tiki taka at Manchester City. It is not a classic tiki taka in Manchester City but the style is similar and he will not need too much time to adapt to the tactics used by Xavi in Barcelona.