There’s nothing to stop them from still admitting their kids even if they end legacy. Just like they will continue to admit kids of major politicians and CEOs, famous child actors, etc., regardless of legacy.
Both of my older kids started doing very small donations ($25/year) to their colleges immediately after graduating because they have been indoctrinated for years that alum to do that and their college friends are too. I guarantee you they are giving zero thought to doing this to increase the odds their hypothetical future kids have a better shot at getting admitted.
BTW, if most Harvard alum are giving because of legacy admissions, that’s not working out for most of them since more legacy kids are denied than admitted. Seems like a risky investment.