Our homes, cars, backyards, etc. aren't likely to be sterilized in this fashion. Plenty of immune challenges to keep our systems busy.

The only side effect of living cleaner so far (although this is still disputable) is not having parasites anymore and not having immunity suppressing effect they have on your system. This still is net benefit though since parasitic diseases take a toll on the body.

problem is immune system has been tuned to develop with parasites and malfunctions without them.

Hence auto-immune diseases.

If this disinfects similar to how sunlight would, maybe it's not so bad

You can balance it out by not being so sterilized in your own home, that something you can control.

Not really since you won't be able to effectively replicate the diversity of a natural environment - an environment like what you'd need to eventually build defenses against.

Why is a food bank warehouse a "natural environment", but a house is not?

Some of the modern wisdom rings true again. Go outside and get some sunlight and exercise. Dig in the garden. Keep pets and hug them. All have benefits for subtle reasons.

You can sterilize a mall at 9am, but it won't be clean for long. There will still be plenty of germs about.