Not even a minuscule fraction of the 4 billion can use Bitcoin which is why I sold out of Bitcoin into Monero over 3 years ago. By the way Lighting Network will not solve this issue. Just read the LN paper https://lightning.network/lightning-network-paper.pdf page 55 where they talk about 133 MB blocks under very optimistic assumptions just to open and close LN channels.
133 mb blocks for 7 billion people all running their own nodes opening only 2 CHANNELS PER YEAR with zero on-chain transactions. Those are some wild assumptions!
My raspberry Pi lightning node has been syncing for 30 days and is only at 85%. We need smaller blocks not larger
The bitcoin maximalists have some serious cognitive dissonance to resolve. The good news for them is their chain is likely to be superseded before they have to resolve these fundamentally incompatible design goals.