I feel like this an argument where you say spring training doesn't matter then you use stats from spring training as if they matter in this case K's and walks. You really can't have it both ways. Regardless, this really isn't a case of my distaste for Baez. It's pretty clear that as hitters both Olt and Alcantara have done better since their struggles than Baez. Like I said, ultimately that might be meaningless in the grand scheme as they were throw away innings at the end of the season and essentially practice. However, I do have a problem if they go out there and say Baez is ready. If Rizzo bats .150 in spring training then fine, it's meaningless because he's already proved what he can do last season. But keep in mind he didn't bat .150 in ST to earn his way onto the major league team initially. After his struggle with the Padres in 2011 where he hit .141/.281/.242 in roughly similar sample size as Baez he went back down after being traded to AAA. He proceeded to hit .342/.405/.696 before his ultimate call up with the cubs. This is what I'm talking about when I feel like players need to earn their way rather than being handed a position. If Baez goes down hits something resembling Rizzo's 284 PA 2013 AAA then despite all I've said about him and his future I'm entirely on board with him earning his shot at the majors again. It's frankly what I wanted to see before being called up last year but he really never reach that level of domination.
Also, I'd like to point out that you bring up hustle, base running and fielding as though it's not something Alcantara has been doing. At the end of the day you carry 25 players. They presently have 7 OFs(Denorfria possibly starts on the DL), 6 IF(cubs.com has Alcantara listed here), and 3 catchers. They have 17 pitchers though they list Turner and Wada who probably start on the DL. It's worth noting if Wada does start on the DL that probably means Zac Rosscup or Drake Britton makes the team as unless they start Wood in the bullpen they have no other lefties. You typically carry 12 pitchers and 13 positional players. So, you're looking at cutting probably 3 pitchers(my money is on Beeler, Schlitter, and one of Britton/Rosscup). They also need to cut 3 positional players. If we assume long term Denofria is on the team you basically have to decide which 2 of the 3 OFs you cut in Lake, Szczur and Sweeney. If you then keep 3 catchers you have to decide on one infielder. That's just the math of things. While it's possible you could cut all 3 of the OF I listed above that then means you're making Alcantara your 5th OF. Your 5th OF sees basically no at bats. For example, Scott Van Slyke last season as the dodgers 5th saw 246 PAs. If that's what you're wanting to do with Alcantara you're entirely wasting him.
In a hypothetical world where Baez is guaranteed to make the team you'd be far better of sending Alcantara to AAA where he plays every day or close to it. There in lies the problem as I see it. Both Alcantara and Baez are good enough prospects that they need every day status. The idea of both works in a world where Olt sucked and didn't play 3B but that seems out now. Baez isn't going to play SS with Castro here. Alcantara isn't going to play CF with Fowler here. So realistically if we assume Soler is an every day guy as well in RF the only possible position for Alcantara to play with Baez is LF but that idea is pretty hamfisted especially with Bryant looming. Playing Alcantara as a back up IF has the same problem as him being the 5th OF. Some sort of 2B platoon gimps both Baez/Alcantara's PAs. La Stella makes perfect sense as a bench guy. He's decent, hits left handed off the bench and he's not someone you're worried about missing a bunch of PAs.
What I see happening is them carrying 3 catchers until they can find someone desperate enough to give good value for Castillo. They leave one of Alcantara/Baez(obviously my money is on Baez) in the minors where they see every day play until that point. At that point, whomever was in the majors will have likely seen enough time to make a judgment call on while whomever was in the minors will have had enough time to show whether they warrant a call up. Though, I should also mention that this roster spot could also go to Bryant.