Effectively single-vendor. I'm not aware of any ACME-compatible CAs that don't have pernicious limits on their free plans (and if there are, I'd love to hear!), and here in the EU we've even recently lost a rather big player...
"multiple vendors, but only one of them is nice enough to give the product away for free" is not "effectively single-vendor".
The other CAs aren't prohibitively priced for anyone who has a business need for lots of certificates, in case Let's Encrypt disappears or goes rogue.
Doesn't ZeroSSL do this? acme.sh has been using it as the default for the last few years. As I understand it, it basically offers the same as Let's Encrypt.
Google Trust Services: https://pki.goog/