> What are the viable alternatives to LE?

None. Big tech intentionally made Let's Encrypt a single point of giant failure.

> And in case none exists, what does it take to build one?

A new Internet and Web standards stack. The whole problem is self-imposed -- we could have published self-signed Ed25519 keys on the DNS instead, and the result would be more secure than whatever it is we have now.

Do you remember the early days of SSL certificates? It took an act of god just to get a certificate: verification rituals like faxing corporate paper work, phone calls, manually reissuing certs because someone forgot the "www", forgotten renewals...

Let's Encrypt is incredible.