Shoehorning the TOS characters into DIS would be a mistake.
Except we're not talking about shoe-horning, we're talking about a situation where the story makes sense to include a cameo or guest appearance. "Sarek," "Unification," and "Relics" come to mind, which while i'm sure started out as ratings-boosters, became great episodes. The second of which happens to be one of my favorite TNG episodes, perhaps even my favorite of any 90s-era Trek. Even Riker (well, Thomas) guesting on DS9 turned into a fantastic episode. The shoehorningRiker and Geordi cameos on Voyager, and Tuvok on DS9 also worked just fine, in my opinion, as they were just sort of throwaway scenes and didn't detract or distract too much, while helping the overal connectivity of the series'.
So in the case where there is an episode that involves them, the question is, do you recast, or use Greenwood, Pine, Quinto, etc? My opinion is recast, for one reason mentioned above: time-wise, they should be 10-15 years younger, and while it's easy to pull off 10-20 yrs older, younger is tougher. Even Pine on STB looks noticeably older than he does in ST09. Combined with the cost of those principle actors, it just is too easy to recast.
That said, my opinion is not terribly strong either way.