My thoughts on Baez are well known in terms of what I think he will be. Excluding them, I think he is. Presumably Russell will be the SS sooner rather than later. Whether it's Castro to 2B, Baez to 2B or whomever that seems pretty safe to assume. Where I would argue Baez is expendable is two fold. For one, if you can sell a team that he is a SS the corresponding value of his bat increases his value pretty exponentially. Even at 2B, his bat holds serious value. This year 2B's across the league are hitting .258/.316/.380 with roughly 11 HRs(per 650 PAs) and 12 SB. Baez arguably can be a 30+HR 20+ SB guy. I'm not as convinced as some on Baez defense but those who like it will tell you he could be good. If we split the difference and say he's average that's a pretty good player. At 3B, that barrier to entry is higher. 3B are hitting .260/.317/.411 with 18 HRs and 6 SB. LF has a similar problem in that you care less about their defense and more about their ability to hit. As such, from a purely value standpoint Baez is worth the most as a SS followed by 2B then probably 3B and LF.
Some would(and have) made the argument if he's a pretty good player at 2B why not just keep him? In my opinion there's several reasons. For one, while his AAA numbers are definitely nice to see the largely mean nothing if they don't translate to the MLB level which we can't say for sure. Additionally, it's difficult to say that Baez will ever be better than Castro has been. You could make the argument that Castro has already proven his value as a hitter and provides some experience that Baez doesn't which has some intrinsic value. Baez outside of Schwarber has the most value of players not in the majors mean if you're going to trade anyone he'd bring the most return presumably.
I'd also throw out the idea that the cubs are ridiculously deep at 2B even if they trade Baez. Besides the idea of throwing Castro there you have Alcantara in AAA, Stephen Bruno in AA(hitting .266/.337/.346), Daniel Lockhart in A+(not that impressed myself but mlb.com has him in the top 30 prospects), Chesny Young in A(.332/.403/.380), Frandy Delarosa in A-(.304/.362/.424 and was also a top 10 IFA in 2012 I believe) and that's before you talk about Happ(heard they are going to try him at 2B in fall instructs) and Torres who's rating at SS in A plus some like La Stella. Now La Stella and guys like Young/Bruno probably aren't likely to be amazing players but they have a decent shot to be league average 2B and they would obviously be fall back options if Alcantara, Castro and/or Torres don't play to their talent levels.
Some are probably scared to trade Baez because "what if he becomes great?" My counter to that is it honestly doesn't matter what he does if you make the right trade. For example, if we throw out a hypothetical trade of Sonny Gray for Baez, Vogelbach, and McKinney plus filler out there does it matter if Baez is a star as long as Gray continues to pitch like he has this year over the prime years of his career? I don't know if that is enough to get Gray but my point here is what you give up really doesn't matter so long as what you get in return is as good. If Gray and Baez both perform up to expectations the only way you regret that is probably if McKinney becomes a star because frankly the cubs probably can't use Vogelbach anyways.
To summarize, Baez is probably the only player who can net you a franchise player outside of Schwarber. I think what makes the most sense is you role with Russell and Castro for the time being as MI be it as is or moving Russell to SS and Castro to 2B. Castro is under contract until 2019 with a 2020 option(2016: $7.8, 2017: $9.8, 2018: $10.8, 2019: $11.8, 2020 $16 mil option $1 mil buyout). Castro has been between a 2-3 fWAR guy with 1 WAR on FA being worth around $7 mil the past few years he's cheap still. From there, you can either wait for Castro to mature or ride with him until Alcantara, Torres, Happ or whomever supplants him. And even if all that talent fails, you're talking about a team which given the young bats they have should be above average hitting anyways. All that being said, I wouldn't move Baez unless you got a franchise player either in their prime or pre-prime.
