Before this turns into too much of a Marcus Smart love-fest, I will first say that I don't agree with the OP - I do enjoy watching him play - but I don't love him as a primary ball-handler or in a scoring role. He should pretty much always be like the 4th option on offense as a spot-up shooter or as a cutter. I understand he has a unique passing ability, but he can't help but try to make the homerun pass. He just takes too many dumb shots and makes too many dumb passes to be a positive in a lead role.
As a defender, he's exactly who you want on the floor in almost every way. I have no idea how he always manages to grab all ball when a defender is going full speed at him. He is also super-clutch on that end of the floor...weirdly so.
I don't necessarily want to trade him, but I wouldn't call him untouchable either. I cringe at the thought of paying him $20M+/yr after next season and fear that's what it might take with a losing team wanting to change their culture.