I think it's more about role than specific roster spot. I think we saw glimpses of what Trotz wants out of Dobson, play a sheltered offensive role with PP time. It's going to take time for the defensive play to be more consistent, but his strengths have been more on the offensive side more similar to Leddy than to Boychuk.
We're stuck with Boychuk for next year unless the NHL surprises us with 2 CBO's. The old man is still a stalwart on the PK, but his ice time has decreased to 3rd pair levels now. Pulock is our horse who plays the toughest minutes and Mayfield is kind of like a younger Boychuk where he is another primary PK'er and helps at ES against the middle lines. It also helps quite a bit that Pelech is such a good match for Pulock and Toews for Mayfield.
Leddy is clearly on the chopping block and Dobson's current skillset is most suited to replace him despite the handedness difference. While we all want to see Dobson on his natural RD side, it is a big feather under his cap to be able to play his off side. Leddy out and Dobson in is the most seamless change in this group.
For next season, I think Dobson is a lock to take Leddy's spot next to Boychuk. The kid has already demonstrated the ability to effectively carry the puck out of the zone or make the outlet pass. He does a nice job making smart offensive plays and has shown that he can get pucks to the net. He got #2PP time as well. It sucks he has to play LD again, but that's the open spot and he fits best there for now.
After next season, I think we lose Mayfield due to expansion (he could be solid trade bait too) and Dobson should be ready to go and move up into his #2RD spot.
I don't see Dobson ever seeing AHL time aside from conditioning stints.