Value is not in a vacuum. Eichel is a a franchise-altering #1C, who is 25 years old, and locked up for 4 full seasons. He likely has the potential to be the best player on the best team in the league. When getting Jack Eichel, teams are absolutely going to be willing to part with "franchise futures" (regardless of risky surgery) because he is and can be a cornerstone player for that franchise.
All due respect in the world to JT Miller, but he is not that, and never will be that. On a really good team, he should probably be the 3rd or 4th best forward. He's versatile enough that a ton of teams would love to add him at the deadline, but every year there are a couple of guys available at the deadline that can bring similar things to what he can.
Obviously, if the Canucks wait a year and he's playing just as well at the deadline next year, he'll get a 1st + pretty good prospect with them likely retaining 50% on the deal. Trading him this year, should be able to get a similar / slightly better package with no retention, as his contract is pretty good, or perhaps you can "double up" -- retain both years, and get packges that include a 1st rd pick plus a recent 1st, or a prospect who's taken some really positive steps towards becoming an NHLer since being drafted.
That being said... just because somebody might give you 2 1sts, or a 1st and a 1st-equivalent prospect; doesn't mean they're going to give you a guy they view as a likely top tier player for them (a la Rossi, Newhook, Byram)