I honestly don't get why people are so enthralled with Miller to begin with. Ok he's a good reliever. Do you know how he became a reliever? Once upon a time the batshit crazy Florida Marlins traded for him, Dallas Trahern (minors), Burke Badenhop, Frankie De La Cruz, Cameron Maybin and Mike Rabelo in exchange for Miguel Cabrera and Dontrelle Willis going to Detroit. He then spent 3 ineffective years as a starter with Florida. He was a top 15ish prospect in the game at the time(as was Maybin). 3 years later in 2010 the red sox traded Dustin Richardson for him. Richardson to the best of my knowledge wasn't a big prospect. After that the sox sent Miller to the minors and rebuilt him as a reliever.
For me anyways the take away in that story isn't that this front office loves Miller. The take away is that this front office loves rebuilding failed high profile pitchers. We've already seen this with Arrieta. They just signed Matusz to a minor league deal. Aaron Crow is also on a minor league deal. It's far more likely in my eyes they go that route and pick up a lessor piece like say Frenando Rodney again.
Andrew Miller fWAR by year
2016 - 1.5 in 33 innings
2015 - 2.0 in 61 innings
2014 - 2.2 in 63 innings
This guy is either the best or second best pitcher in baseball per inning this year with Kershaw being the only guy who can match that. To call him a "good reliever" would be like calling Mike Trout a "good hitter". If you want to argue that relievers are overrated that's one thing but to question how good Miller is doesn't make any sense.
Unless you think something is going to regress dramatically after two+ years, I'm not sure why anyone wouldn't want Miller. And in a playoff series, he's far more likely to sway an outcome than any normal hitter would. In the course of a regular season, Schwarber gives you more value since you're only talking about 70 innings of pitching vs 500 PA; however in a short series the value tips to Miller as you'd rather have seven innings of 1.5 FIP pitching in extremely high leverage situations versus 20ish PA for Schwarber.
The other thing to remember is that the Cubs have the players to replace Schwarber's value (i.e Baez, Soler, Contreras, etc) without them missing a beat where as the Cubs simply do not have a pitcher they can acquire that they have multiple years of control who comes close to Andrew Miller's value.
I wouldn't do it straight up but if I got back a rotation arm for 2017+ (when the Cubs will have a need for cheap arms to offset a new Arrieta or a new Arrieta replacement deal), I'd do the deal.