Since proper email transport encryption is finally getting some momentum with DANE and MTA-STS, I'm surprised that STARTTLS Everywhere doesn't mention them.
DANE is being recommended (mandated?) by the European Union, and is on the rise. MTA-STS is backed by Gmail, Microsoft and many others, which is likely to give it traction.
To sum up from that post, we think STARTTLS Everywhere is a stop-gap measure until DNSSEC is fully deployed, and STARTTLS Everywhere can act as a preload list for MTA-STS (to prevent DNS downgrade attacks).
The draft is very clear about DANE being preferred from both a security and deployment perspective (does't need to get certificates for all hosted domains).
Sure, if DNSSEC fail, then SMTP-STS is better than nothing.
I'm pretty sure that the big providers could have had deployed DNSSEC relatively easily, if they just wanted to.
I think other DNS-based email security features such as DKIM and DMARC motivates DNSSEC as well.
DANE is being recommended (mandated?) by the European Union, and is on the rise. MTA-STS is backed by Gmail, Microsoft and many others, which is likely to give it traction.