I don't think the plan is to just "dump" or "move on" from Miller.
Good, young centers are almost impossible to acquire. Last guy that sort of fits the mold was Kirby Dach and he was pretty bad at center, and might not be a long-term center. Most teams want at least three good centers on their team... there's only so many centers available across the league.
I think they are pretty comfortable with JT Miller's contract (as they should be - he likely has 4-5 good years left before the contract might be a problem). They signed it so one would hope they are comfortable with the deal. I don't think their plan necessitates that they trade JTM.
I think they are OPEN to dealing him if the right circumstances align. As you mentioned, it's going to be incredibly difficult... that's why the deadline deal was hard to orchestrate. I think the rationale is that Pittsburgh will likely offer the same deal to Vancouver in the offseason.
The long-term deal won't really be a big deal for PIttsburgh since the will be rebuildling in 4-5 years after Crosby and Malkin slow down. 2 1sts + Granlund is still a fair offer. We can try to flip Granlund with retention or keep him as a 3C.
Need to parlay those picks for a young 2C. Not many on the market...