No one buys a single nail, they're either sold in small packs of a bunch of them, or by weight. Any single item that is too cheap to be less than 1 sat will get bundled up in quantities that allow it to be conveniently bought, or there is some sort of other accounting system in place, (something off-chain like a credit system or account balance or something).
Rice is sold in bags. The smallest retail quantity I've seen is maybe 1 cup of rice. Liquids are sold in bottles or cartons or jars.
Arguably, that's a function of the non-viability of microtransactions using current tech. But even today, micro-transactions is a use case. People sell e.g. single cigarettes, and people buy them. Can you today sell single cigarettes using BTC on-chain? Not really.
I'm not saying people will buy single cornflakes if they suddenly get the chance, just that if you only want 1 nail, then that's what you'd prefer to buy, even if the rate you're paying is 5% the price of 100 nails. Both parties are happy; the retailer gets that warm satisfaction of a 500% markup, and the customer doesn't have the problem of holding on to 99 nails they don't have a use for.
If a currency or payment system existed that made that economically feasible, the Bitcoin network might lose that business to it (lightning channels could handle it, but they're denominated in subdivided satoshis in order to do so, which was my original point after all)
It's unlikely we will see anything smaller than 1 sat even if divisibility becomes an issue. The surviving altcoins will provide liquidity for small ticket items, but the existence of those very same altcoins will be volatile.
You're thinking about this as if the current dynamic of small coins being "silver to Bitcoin's gold" can persist indefinitely, yet there's already a possibility of that relationship breaking down. Coin devs won't stop coming up with new ideas, so it's puzzling to me that you've become so set on the way this market worked back in 2013.