See, I don't think KW vs. Smart should even be the conversation. They don't work together that's true, but all the negatives I see growing in Smart don't really have to do with Kemba, it's just the Kemba pairing makes them worse. Kemba isn't the reason Marcus is struggling against quicker guards this year, he definitely isn't the reason he is a well below average shooter, Kemba isn't why Marcus turns the ball over way too much on moderate usage.
I also don't think Kemba's contract is that big an issue... he's still a good offensive player, and he's not a terrible defender, just a below average one, and there are only 2 years left.
I think Kemba vs. Marcus misframes the question in part because it frames Marcus as a starting PG option.... he shouldn't be. He's a SG if we're being generous, and undersized swing if we aren't. You can't plan a roster around a starting PG who can't stretch the floor, is a servicable distributor, turns it over a bunch and whose best attribute is his switch defense on strong 2-4 players.
I mean, I look at Kemba, and I see a player who is a much better shooter, better passer, turns it over far less on much higher usage, rebounds better (that's just embarrassing for Marcus), and is generally a much better offensive player. He's not as good a defender, but Marcus seems to be slipping on that end, particularly against PGs.... which is concerning if that's who you want him to play.