There's another likely scenario (at least in my mind) where Bitcoin is a global reserve "currency" like gold used to be. In this instance, the on/off ramps that exist already would still likely be the ones around. That is to say, completely centralized instiutions that interface with completely centralized fiat currencies. Bitcoin isn't going to replace countries, countries will likely have fiat stablecoins which is exactly where our increasingly cashless societies are headed towards.

tldr; either the crypto itself is traded like real money, or a l2 or even centralized unconnected layer is - either way on/off ramps that currently exist (p2p and centralized) continue to exist and thrive.