alt Hacker NewsIt's not single-vendor. The ACME protocol is also supported by the likes of GlobalSign, Sectigo, and Digicert.
You've got to remember that the reduction to a 45-day duration is industry-wide - driven by the browsers. Any CA not offering automated renewal (which in practice means ACME) is going to lose a lot of customers over the next few years.
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...