The actors literally have no choice since they are contractually obligated to film another movie and Disney is filling their pockets.
I get Chadwick's death was too early and out of nowhere since he didn't tell anyone about his cancer, but Black Panther is a fictional character. It's an unfortunate circumstance that Marvel needs to work around. But the Black Panther dates back to 1966 and there have been many iterations of him - Chadwick is just one of those iterations. Stan Lee died within the past few years. When he died did people say to stop making Marvel movies because he created a ton of the characters? Nope.
Obviously it's not exactly the same, but my point is, his death is unfortunate, but we also need to move on. Do you think Chadwick would've liked BP2 to be cancelled because he died? We'll never know, but I highly suspect the answer to that is no based on how he conducted himself.