The comparison is dumb for a number of reasons but (while heresy to say out loud) Lewandowski is quite a bit better than Sergio at present, and has been for some time. He's scored double the amount of goals over the last five years as Sergio has and is way higher on both a per 90 basis and xG.
According to Sergio's dad, Sergio wanted to renew & Pep didn't.
So whats the point of letting him leave and then go to pay transfer fees + high salary for another 32 years old striker ?
When Sergio is fit, he is as good as any striker out there ( Apart from Leo )