Eh, I don't know, there are a lot of scoring guards that play(ed) passable D at least. This year look at Booker for example. Obviously some of the guys like Jordan or Kobe, but they are superstars and Mitchell is not close to that tier. Or Gary Payton, for a PG that could do both. Even some players not necessarily on the top but solidly in the mix such as Lowry and the guy I wanted over Conley, Brogdon, who would have been a stellar running-mate to Mitchell. Holiday fits that mold too. Even like a LaMelo Ball and Lonzo are better ball-dominant wings with better D than Mitchell. Don't even get started on Curry, who is a plus defender, and Thompson, a stellar scoring wing and defensive player.
They do exist, and some have shown they can step up this part of their game. I think Mitchell is just too small and doesn't have the skill-set, nor the desire, to be a strong defender. The desire part is the worst. Look at Caruso to see what desire can get you, as he is in no way the superior athlete nor has the strongest skill-set, but his desire and effort have made him one of the best perimeter defenders in the league. Of course he doesn't have the offensive burden, so look no further than Booker for a great apples to apples comparison to Mitchell, as Booker has really stepped up his D this year and it has paid big dividends as the Suns are very likely to win it all this year.
I don't expect Donovan to be the scoring champ and the DPOY, but he could at least improve enough to kind of bother someone like Brunson, who is not known as a scoring powerhouse. But he plays matador defense, along with Conley, which allows record nights from these guys.