I understood your assessment; its the conclusion that is incorrect.
The problem is that it takes significantly more delta V to make the inclination change from [the theoretical] Baikonur 46 down to 37 (or some proposed middle ground inclination) than you gain by launching from a slightly shallower inclination from The Cape. To make that inclination change from Baikonur you have to wait until you're on orbit where you're moving WAY faster than the surface of the earth, so any directional changes you make require a LOT of energy.
Putting real numbers on it (and assuming I didn't make any errors in my super quick math) you gain 48m/s by launching at 37 degrees from The Cape instead of launching into the theoretical Baikonur 46. Conversely, launching at 46 degrees from Baikonur and then turning into a 37 degree inclination once on orbit requires you burn additional 1.19km/s. So...for your middle ground 37 degree ISS orbit, you waste 1.19km/s for every Baikonur launch to gain only 48 m/s for every Cape launch.
Mathed a different way, your "roughly" ideal inclination is ~44.5 degrees, at least when you just consider delta-V. Obviously that doesn't take into account mass, but if we were to consider the history of ISS launches which are overwhelmingly Russian, total mass launched would certainly push that ideal inclination even closer to Baikonur.
Of course, all of the above means absolutely nothing when you factor the minimum practical inclination of 51.6 from Baikonur.
