I do think it's pretty damn telling that in my hypothetical, folks would be willing to pass on arguably one of the best left handed hitters of all time, in his prime, just because there is a need at SP. Yes, even if they have a chance to acquire a stud pitcher with control, I'm still picking Alvarez. (And yes I know this is a wild hypothetical with almost no prospects of becoming reality - my whole point is that if the Astros did make him available, the Cubs should be aggressively pursuing him). He'd instantly be the best player this team has had in like 25 years. Given how much the Cubs seem to love former Astros players it would be weird if they hypothetically made no effort to get the guy who has a .1070 OPS.
There are good free agent starting pitchers every single year. Whereas a guy like Alvarez almost never makes it to free agency and the Cubs sure as hell aren't good enough at drafting to find a guy like that organically.