You mistake is, you overestimate the amount of people who want to be self custodial. You don't need to onboard every human being in the world on-chain.
Given the US example that would be several years in the absolute best for lightning case to onboard even 5% of individuals. Lightning is doomed from the start.
And if you don't care about self custody then the overhead of using a blockchain is a waste.
It is not black/white. It is okay to have the freedom to become self-custodial anytime, but not everybody needs to transact in self-custody all the time.
Taproot was another major step that enables lightning upgrades in future versions (such as zero-fee channel opens) that is barely discussed. The number of X years for onboarding Y amount of people is not accurate, as it disregards all major developments of the last 5 years.
You need enough users for providers to bother making it an option to pay with so it really is black and white. You might get a few niche providers offering it as a payment method without a critical mass of users but most companies aren't going to invest time and effort into implementing a payment system a tiny percentage of users have access too and if I need to trade money with my friends the low % means that in the vast majority of cases they aren't going to have lightning available either.
And if you don't care about self custody then the overhead of using a blockchain is a waste.