I'm guessing Elder doesn't get more than a handful of kids from Harrison. It's not like all of these kids will stop going to Elder if they were to happen to beat them. None of them are currently going to Elder now, outside of 1-2 per class, and those are likely legacy.
Outside of Elder cutting a check to play at Elder every year, there's no upside at all for Harrison. They're never going to beat Elder. It's even worse than the Oak Hills pipedream of one day beating Elder. Likely won't happen in our lifetimes.
Elder homers think everyone should be entitled to play them, but it has to benefit both schools. This isn't charity work.