What if Lightning isn't going to be the killer protocol that consumers directly interact with in order to make their payments online? What if, just like the blockchain itself, it simply winds up being a piece of a settlement layer that other things are built on top of?

Would that be the end of the world? Would that be a failure of Lightning? I would argue absolutely not. From the very beginning of development on Lightning it was incredibly clear what its scaling limitation would be. The whitepaper literally brings up the issue of not getting support for softforks needed in the future as a limitation of Lightning's potential scalability.