It would take 27 years to onboard every internet user to the lightning network unless you start adding level 3 aggregators and then at that point you lose all the benefit of it being on chain at all.
It would take almost 2 years just to onboard every American assuming during that time there were zero other bitcoin transactions. Then you need to add the fees for the on and off ramps to the individual transaction fees to get the real cost per transaction noting that these would go up quite a bit as the competition between lightning and non-lightning uses of the transaction space would drive prices higher.
Most cryptocurrencies have an adaptive block-size mechanism which allows the blocks to grow to a reasonable size which could facilitate such an onboarding of users. So it isn't a technical problem, it is just a question of bitcoin's current leadership, which is controlled by companies like blockstream.
Even if you increase the block size 100x though you're still not improving the numbers much since my very generous numbers ignore activity outside of lightning and assume a single on chain transaction for every user and a perfect network.
Maybe your understanding of lightning is wrong here. Yes you open channels, and transactions in lightning need open channels, but you do NOT NEED to open channels for specific transactions. You open channels once, and transact over them for years. I run a lightning node with more than 15 channels (each to different lightning nodes) that are all older than 1 year (I route payments, so I have way more channels than needed). You can batch-open channels, i.e. I could have opened all my 15 channels with a single on-chain transaction. Taproot update would make those "commitment transactions" onchain way smaller (in byte) than needed to in the past.
Go read what I've said again. My timelines are based entirely on each user making a single transaction(opening a single channel).
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.