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.
And if you don't care about self custody then the overhead of using a blockchain is a waste.
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.