I do believe that Pep would prefer Sanchez to Sergio, but if Sergio were sold then surely that would necessitate bringing in another striker, which I find unlikely given the current spending. I'm also not buying the Aguero to Chelsea rumours. If he were to be sold then it certainly would not be to Chelsea.
However, if we're talking about Mbappe, then I can certainly see Sergio being sold. First to bring in some funds for the inevitable world-record fee, second to make room in the wage bill and third to materially guarantee him game time. A front three of Sane Jesus and Mbappe would be amongst the best in Europe for the next decade and worth taking the risk of having only Sterling and Bernardo for rotation.