Yeah, it's a bit far fetched but after Cloudflare CEO basically threatening to cut off Italy I was wondering what would happen if US really invades Greenland.
A simple windows to linux migration is not enough. If certificates expire without a way to refresh you'd either need to manually touch every machine to swap root certificates or have some of other contingency plan.
Remember that there are lots of CAs, and quite many of them are based outside of the US. Those CAs currently do not offer ACME services for free, but there’s nothing stopping them from doing so.
I would say that the WebPKI system seems to be quite resilient, even in the face of strong geopolitical tension.