I'm a little confused by the "10,000 years is plenty of time" argument, particularly when the crickets in Hawaii or lactase persistence examples are used as evi

I'm a little confused by the "10,000 years is plenty of time" argument, particularly when the crickets in Hawaii or lactase persistence examples are used as evidence. To me, these were relatively minor evolutionary steps. I'd even go as far as to question whether crickets changing their singing might not be explained by culture or learned behaviour — at least partly? As for lactase, is it such a vast leap from drinking mother's milk in infancy to drinking the milk of other mammals? Comparing these examples to completely rewiring the human digestive system, insulin response mechanism, and immune system to deal with digesting and utilizing foods (refined sugars, grains) in vast quantities which we rarely if ever encountered before seems like something that might take a bit longer to evolve. The argument almost seems to say "evolution has been observed on shorter timescales, therefore any evolutionary leap is possible on shorter timescales."