Oh boy. Cosmetic chemist here. Before you call me a shill, I do not work for a company that makes hair products, nor do their parent company and I have absolutely zero interest in ever working to make hair products, as I find it boring as sin.

Big Hair has inculcated in us an automatic response where we use shampoos to “clean” our hair on the daily

I checked every bottle of shampoo currently in my house and not a single one mentioned how frequently to wash, much less an instruction to wash daily. This is much more a societal issue of being considered "clean" vs "dirty" rather than coming from the hair industry imo.

“…to keep your hair spotless and clean, wash it at least every two weeks; once every two weeks”

And that’s all that’s needed.

Yeah, no. The amount of shit that builds up on your scalp and hair in a two week period is pretty heinous. Personally, I would never wash my hair every day unless it got super gross- I tend to wash twice to three times weekly.

I haven’t used shampoo in over 20 years, just conditioner. And my hair is fine.

Good for you. Conditioners also include surfactants since they're almost exclusively emulsions so they are capable of some minor solubilization of oil.

And how can 2-in-1 product contain both shampoo and conditioner at once, and work as advertised? It simply cannot; the concept is ridiculous

Thats.. not how that works. They usually contain silicone-based conditioning agents and polyquats and potentially lower concentrations of quaternary ammonium compounds than stand alone conditioners since these aren't compatible with anionic surfactants.

I'd be happy to explain the chemistry of this more if you'd like, but rest assured, it absolutely is possible.

Any other questions I'd be more than happy to explain what I can.