I found out a few years ago, when I had a nasty scaly seborrheic dermatitis on my face (and my dermatologist's recommended cream would have cost me more than $450 as co-pay!), that warm salty water actually took care of the problem. Once that was over I wondered the same thing about the bio flora, and if perhaps that might have been the problem from the beginning. So, since then, I wash my face everyday with just water (and moisturizing with coconut oil). I take a shower twice a week or so (my hair doesn't need washing more often than that), and other than shampoo I only use soap on the private bits, and occasionally on my face. I use a washcloth, so friction also plays a role in keeping me clean. I have no idea if the skin flora is helped or hindered in any way by this, but I feel fine, and my skin is clear. And, some years ago, I was washing my hair with baking soda, and rinsing with ACV, and my hair quite liked that routine. I have got out of the habit in recent years, but might consider going back to it.
ETA: And, of course, I use soap when I wash my hands, or if particularly dirty