I assume you have evidence of the claim that by being regularly sick your immune system is stronger? Sure, it's run into more things it has learned to fight, but just because it knows how to fight a catalogue of things it doesn't mean that it will be better with the next novel pandemic, right?
The risk is more around autoimmune conditions like asthma and allergies. If the immune system doesn't start fighting off a variety of pathogens from an early age then it might go haywire later.
I don't much care for the hygiene hypothesis for allergies. I grew up on an acreage with all sorts of animals. I'm allergic to all of them, so much so that my parents got my allergy shots for several years. I'm most allergic to horses, which we had a trio of. It just doesn't make a lot of sense to me. And if it were true I'd much prefer more allergies to COVID.
Of note in that paper they are suggesting therapeutic strategies that involve controlled exposure of particular items, and how those exposures are fraught with potential dangers as well.
