You asked for justification. You asked some questions. People answered them. Yet you attack people and criticize programs. I'm not even saying I disagree with everything you're saying, but you have a real habit of asking questions and then when you get an answer you don't agree with you just like to attack rather than listen to another perspective or engage with civility. It's a pretty big turnoff. Your quickness to criticize GBS (something you've done for the "full decade of mediocrity" that GBS has had) also suggests some deep-seated insecurity about Loyola's success not being 100% due to good coaching and hard work...
To the questions at hand - yeah, the number of kids "lost" to Loyola seem to be pretty consistent. And as other posters have pointed out, it's the idea of upgrading at 4-5 positions that can make a team go from 3rd place in conference, to winning playoff games, to deep playoff runs. Anyone who has coached knows that to be true. Loyola typically has a lot more depth than many of the nearby public schools and can afford to "lose" one kid to GBS from a family with Loyola roots and/or adjust to injuries to key guys during the year. That's NOT to say that GBS or New Trier or other neighboring schools can't "recruit their hallways better" or improve the culture in other arenas, but when talented/big/athletic kids go to Loyola who otherwise might have attended the local public school - it certainly has a tangible impact on their immediate ceiling.
Sure, maybe those kids choose Loyola because of their football culture and winning and whatever else. Kudos to Loyola for building all that and having a culture that draws in kids. But the point is - not all schools have an equitable playing field in terms of building towards success. You can dismiss it all with a "boo hoo, big school, try harder" attitude, but that's missing a lot of the nuance. Maybe they can try harder, and maybe Loyola's pretty extreme success down the road or around the corner ALSO makes it harder to turn that leaf over to a new football culture. Those things can both be true.
And you can keep pointing to the Noll era for GBS, but again that seems a little too simplified for a poster like yourself. GBS had under 5% students qualify for free and reduced lunch during his tenure and now have nearly 20% qualify. The hispanic population at the school has more than doubled. And maybe most importantly, the week 3 and 4 games which always used to be nearly automatic wins against the CSL North for the CSL South teams have been replaced with annual games vs either Fremd & Palatine or Conant & Barrington. Waukegan, and then Niles North, have been replaced by Glenbrook North (the most competitive team in the old CSL North). These changes in crossover games have hurt New Trier, Evanston, and GBS's records over the past few years and all three have found themself hosting a lot fewer playoff games in the time since. I know you've pointed out that schools with more diverse populations can still win - and that's true. But, again, all schools are different in different ways and to just hammer the idea that "well he did it once, see" is missing a lot of the variables that go into a successful football program.