That's the key question: If the Bitcoin evolves into a circular one, where spenders earn most Bitcoins they have (and do not buy them at exchanges), then - and only then - Bitcoin can also evolve into a currency.

LN can be a crucial element here: Lightning is a tool which is able to facilitate some purchases, and once it becomes established there could be a "domino effect", encouraging spending in a very similar way to what prepaid cards achieve. The coins you have locked one time on LN, are coins you'll be willing to spend eventually. Thus, businesses for the first time have a tool to measure the potential of the "Bitcoin as a currency"-economy. If this economy is growing, then there will be more businesses trying to enter that market, which means more people earn their money in BTC. And so on - the money could start to really circulate, instead of being mainly bought and sold at exchanges.