I do agree with your point that bringing in good sportsmen specifically on their sporting ability to fill perceived gaps in teams will adversely affect HSC results.
As a matter of practicality you are right but, in its own way, it is a defeatist attitude that enables the jocks.
There is no connection between being a good rugby player (or any other sportsman) and not being able to get a good HSC mark. (Good means close to potential)
Listening to blokes a lot younger than me the problem comes with the academic slack that is cut to blokes in the various 1st teams - they are forgiven all their sins because they beat some other GPS school, won the HOTR or whatever.
The problems this creates can be seen in some pretty high profile Wallabies and former Wallabies - one of whom thinks that a string quartet has an abottoir they perform.
Frankly, if I was running the U20's program I would want kids who have shown they can multi task.
Besides all this: better educated players make better players because they tend to be more coachable, in my experience.