As long as Morrow can learn to be better than TDA in his own zone, he’ll be fine. You have to think the plan is for him to be the Burns replacement eventually if all goes well. So, he’s likely to play with Slavin if he hits that low bar in the coming years.
Slavin probably still has 3-4 more years of being able to cover for his partner in him.
Personally in 2yrs, this is how I see the D:
Slavin-Morrow
Nikishin-Walker
Gostisbehere-Chatfield
By then, guys like Badinka, Fransen, etc should be knocking on the door, ready to replace Chatfield and Gostisbehere.