I don't know much about this stuff, so apologies if this is a silly question:
If you needed to revoke all the certificates, couldn't you just revoke the handful of intermediary certificates and call it a day? I assume you'd want to revoke them anyways if there's a situation severe enough that merits revoking 200 million certificates.
We have a set of backup intermediates that can be activated if we had to revoke the active ones for any reason, so the disruption wouldn’t be too high hopefully.
(I work at Let’s Encrypt, but this is my own opinion and not that of my employer)
There is no "handful" of intermediate certificates -- there are precisely 4 (for Let's Encrypt [0]) and they are essentially on-line root certificates. And if those certificates aren't even compromised, revoking them would only harm the ecosystem.
They're essentially on-line root certificates because they serve most of the function that such roots would serve if they were allowed. There aren't a bunch more available to replace them, so this means recovery now requires a key ceremony, figure on a week to a month to arrange that.
Whereas if you're able to "just" revoke 10 million end entity certificates you can recover immediately.