In summary: He stopped washing his hair with any products and started using just cold water, in an effort to help with his dandruff problem. Overall seems to have worked for him. Seemed mildly interesting to me.
Not actually NSFW, but I tagged it because the thumbnail is a close-up of some pretty severe dandruff, and there’s a few seconds of video in the middle where he shows the condition of his scalp before quitting shampoo. Just thought I’d mention, in case that makes you squeamish.
You are correct about how our skin has these natural defenses that are much more sophisticated than most people are aware of, but semi-regular hygiene is still important to clear off the debris left behind by this ecosystem, as has been known for some thousands of years. Once every couple days to a week is ideal for most people depending on factors such as humidity and personal biology.
Exposure to “everyday contagions” can greatly strengthen the immune system without making us sick as well, but not always, and the behavior of primates and young children eating their little green nose warriors is actually believed to have evolved for this benefit. Exposure has become more of an issue as civilization has grown in size though, and contagious diseases are being spread simultaneously from multiple points in the global population with more opportunities to find their way into that circle.
If you have a lifestyle that involves regular contact with other people, you may want to be washing your hands frequently.