As much as there are some posters that can never be educated, there are several people here that do a terrible job explaining their position and why it is there position. Some information that I think I understand from looking at swinging strikes (I'm not very smart so I could be wrong), is that there are ways to be successful with a low swinging strike percentage. Mark Buehrle is a great example (better than Alvarez considering how long he has done it for, others I can think of off the top of my head are Fister & Colon)...so in follow it's interesting to look at what Buehrle does well to be successful;
1. High groundball rates. Sanchez gets a check here too
2. Great fielding. We'll give Sanchez an incomplete here.
3. Low walk rates. This is where Sanchez has to get to...and this is where Sanchez's minor league history comes into play.
4. Lower BABIP. Buerhle's best ERA seasons are tied directly to his lower BABIP seasons. This is really out of Sanchez or anyone's control but it's a factor for short term success for Sanchez. A great example of a younger guy that got low swinging strikes that had good ERA based success last season is Shelby Miller. His swinging strike % came way down last year and his BB rate wasn't great still. His .256 BABIP was a huge factor in his 3.74 ERA. I'd also like to add that the Cards felt that Miller's best value was in a trade. The same stance that JFAs has openly talked about with Sanchez.
So no one (not Tercet, not JFAs, not BTS, not me) are saying that Sanchez can't have success. But in a season where the Jays will likely need a successful starter, chances are he will need one of the following to happen:
Develop a pitch that generates a higher swinging strike percentage (it's possible but that is something that is more likely to happen in the minors) or make changes to his existing pitches (higher velocity, more movement) and increase the Swinging Strikes.
Lower his walk rate from his minor league rates. This one is the biggest key, can he do it? History says no, but it's possible that in the minors his walk rates were directly a result of the Jays trying to get him to do things he wasn't comfortable with.
Get Lucky! If he's going to have low BABIP related success it's fine for short term results which is what most here care about. It still doesn't solve Sanchez long term though.
I hope I've covered things in a way that explains why people are struggling to see Sanchez having success, but he's still a top prospect.