For me much of it comes down to the club's evaluation of Havertz - if Tuchel continues to play him as a false 9 and wants to pursue players to fit that system, then maybe Sancho makes some sense especially if others leave - Ziyech perhaps? That would leave us with Havertz, Werner, and probably Mount as potential options as the 9 in attack - not terrible all things considered.
Also would say it's worth considering the last time the club viewed a talent as too good to pass up without considering squad fit - that'd be Havertz. Certainly there are legitimate reasons why he's struggled (e.g. severe case of COVID he took a while to recover from, 21 year old moving abroad and unable to see his family and donkeys for months on end, adjustment to the pace of the PL, etc.), but I think it's fair to say he hasn't hit the ground running. Some of these considerations might not apply to Sancho given his roots in England but might give Chelsea pause.