Long before Let's Encrypt, SMTP transactions with STARTTLS have permitted self-signed and non-root-CA chained certificates.
The pervasiveness of self-signed certificates for SMTP servers means that rejecting them would drop large amounts of email. STARTTLS is basically useful for thwarting passive collection of network traffic.
Gmail's new rules on unencrypted e-mail don't support self-signed certificates though - you have to use an offcial CA-issued certificate from one of Google's approved CAs.
That doesn't bring a lot of extra security though, because there's no name verification. I can get a valid letsencrypt cert on anyrandomdomain.com, and if I can hijack your MX and point at it, it's "valid".
I don't need to control your domain. If I control my own domain, which could be any throwaway domain I just purchased, I can get an SSL certificate on it.
And if I can point your MX records there, via hijack or any other means, then I have a valid SSL certificate for receiving your email.
Yes, they recently enabled the DNS validation. Otherwise, it wants to use a webserver to validate ownership. It can spin up an embedded webserver if you don't have one already.
I would say the opposite: There is no excuse for cementing the role of CAs for SMTP. DANE doesn't need any CA, and there is no problem with legacy clients that require CA-signed certs.
It's been a long time since the cost of a cert was an inhibiting factor - at least for 1st world businesses. (I don't suppose I'd want to be paying for a US dollar priced TLS cert in Zambian kwacha or trying to get a petty cash reimbursement for one on a typical IT salary in South Sudan...)
The pervasiveness of self-signed certificates for SMTP servers means that rejecting them would drop large amounts of email. STARTTLS is basically useful for thwarting passive collection of network traffic.