Quoting: MisstheWhalers
Every team needs more 1sts, Wheeler has one year left while Miller has like 7, this would get Van out of that contract and a 1st towards the future.
Miller and Wheeler probably block it anyway and not much reason for Winnipeg to do this, they needed to trade for Miller when Van did, not now though.
If you are counting this season, Wheeler has 2 years left just like Pearson and Miller has 8; however, Miller is 7 years younger than Wheeler, producing more, and his contract will probably age similar to Wheeler's. So, it's like you just want the mediocre/bad years of Miller now to save 4m (not sure for what exactly) + a 1st.
Vancouver has their own 1st, I guess they could use more if their plan is to just draft a ton of RHD. Otherwise, they have a full top 9 on the NHL-level, and it's not like their prospect pool is completely barren either.
If Vancouver needs cap for something, they are better off just attaching a pick to Pearson, rather than downgrading Miller to Wheeler. If they trade Horvat, that's 5m, right there.