Thanks for this thorough response; from this and other comments I see that I wasn't factoring in all the ways in which LN necessitates on-chain transactions. As you point out, the security budget has so far been more than sufficient; I'm just trying to game out if a major government decided the Bitcoin network was an existential threat, but it's hard to predict developments on the scale of decades. The replies here have at least convinced me that LN adoption isn't an obvious future vulnerability at present (which I figured but wanted to be able to explain why), and that there are options should it trend toward being one.