the last mile problem is how you move on-chain coins into off-chain and back. even if LN works great, with liquidity providers, reliable paths, watch-towers, and splicing, as an end-user you still need to move between the layers every now and then. you still need to splice in/out to top your account up or down and that's not something LN can solve ("inbound liquidity" is the common name for this limit)

theoretically only 500,000 splicings can be done every day and that's still not enough for 1 billion people, let alone 8

with covenant an entire new paradigm of optional UTXOs can be enabled through coordinators who control the on-chain funds, where in to optimistic scenario you don't need the chain to move between these off-chain "virtual" UTXOs while pay and getting paid