They are not in control of the US president.
You could argue that The Don in charge of the US is in control of letsencrypt
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.
I would say that the WebPKI system seems to be quite resilient, even in the face of strong geopolitical tension.
The far bigger problem is the American government forcing Microsoft/Apple/Google to push out a windows/iphone|mac/android|chrome update which removes all CAs not approved by the American government.
Canonical/Suse may be immune to such overt pressure, but once you get to that point you're way past the end of the international internet and it doesn't really matter anyway.
He's not in control of letsencrypt or any other US-based CA.
It may not be well known, but Trump's administration loses about 80% of the time when they've been sued by companies, cities and states.
There's much more risk of state-sponsored cyber attacks against US companies.
It would be nice to at least have a very high level contingency plan because in worst case I won't be able to google it.
You can also read a lot of anti-Trump articles and comments on countless web-sites, some under .com and some under other top-domains. As lunatic as Trump is, he hasn’t shut that down.
“Is the TLD root management really split up vertically”
AFAIK, yes, it is.
But if the global DNS would somehow break down I guess you either have to find an alternative set of root servers. Or communicate outside of the regular Internet. Such an event surely would shock the global economy.
The majority of people use their own ISP or an anycast address from a US company (cloudflare, google, opendns). Quad9 is European.
However any split in the root dns servers signals the end of an interconnected global network. Any ISP can advertise anycast addresses into its own network, so if the US were to be cut off from the world that wouldn't be an issue per-se, but the breakdown of the internet in the western world would be a massive economic shock.
It wouldn't surprise me if it happens in the next decade or two though.