nobody I've ever known personally who has invested in Bitcoin or other cryptos actually physically hold their own private keys.
Meet smarter people, then. I would say maybe you'll learn something, but you seem to enjoy performing a routine where you pretend not to understand these things.
I feel like this is good advice. I don't know a single bitcoin investor who does not hold his private keys, although most of these people are the ones that I have talked to about bitcoin and directly or indirectly pushed towards it. You need to broaden your horizons, OP. How many bitcoiners do you really know in person? Who are these people and how much do they own? Don't take this as an insult, but if they hold a few hundred dollars worth of bitcoin, they aren't serious about their investment, so I wouldn't expect them to be serious about concepts like self-custody, decentralization and security.
The fact is that if these "stupid" people you are talking about--the ones who don't hold their private keys in their own physical possession--all dumped their Bitcoin holdings, then the price of Bitcoin would plummet.
I've encountered people who have a lot to say about these things and tell me how I should secure my bitcoin and then I ask them how much they have and they happen to own 0.01BTC and my response to them is that I wouldn't care where my private keys are if my whole investment was worth less an average mobile phone, or a set of car tires.
the technology has no other viable uses because it will never scale and doesn't solve any real-world problems,
What real problems would you like it to solve? Poverty? Wars? How about dilution of money supply by the Federal Reserve and inflation?