[schoen]

I don't know who has what legal remedies when a nonprofit acts inappropriately, but another observation is that most of Let's Encrypt's technology is developed in public.

https://github.com/letsencrypt

If you needed to set up another ACME-compatible CA on the same model (which could then be a drop-in replacement compatible with the existing client base), it would be a lot less expensive (although it would require datacenter build-out, hiring an operations team, and a variety of PKI-specific stuff like key ceremonies, HSMs, cross-signing, CPS, and audits).