James McCann was better in 2019 than he had ever been in his career. He was very good and deserves respect but there are plenty of reasons to believe that he may regress in 2020. That is not to say that he will be bad but to think he will be an All-Star catcher again next year is a little far fetched. His value may never be higher on the trade market so trading him right now might net the White Sox the most return. If they were to wait on him and he regresses in 2020 then they might see that as a missed opportunity to move him while his value was sky-high.
McCann is a good hitter and a good defensive catcher but you can’t expect him to be elite again. His .273 batting average in 2019 was his career-high and the next closest was when he hit .264 with the Detroit Tigers in 2015. He was also 25 years old in 2015 so going into his age 30 season you can’t expect him to get any better. He also tied a career-high in games played with 118. His batting average in 2019 wasn’t his only career-high either. His 18 home runs, 60 RBIs, 120 hits, and 62 runs scored were all also career-highs for him.
This is not to say that McCann can’t be good for the White Sox or anybody else in 2020 because he surely can. He could even prove everyone wrong and repeat what he did in 2019. The only thing is that his trade value might never be higher in the future than it is right now and a team might be willing to overpay the Sox as a result.