Of all the attackers you could worry about, I wouldn't put your own email hosting provider at the top of the list. MTA-STS is nice, but it's hard for an email client to guarantee that the hop-to-hop links between itself and the recipient's inbox are going to be protected with TLS. Passive snoops between mail providers seem like a much more realistic threat.

Also, by putting a public key in the public DNS, it means that Alice can catch her mail provider's DNS server giving false information about her public key, which would come with cryptographically signed proof of which entities were involved in that lie. Ideally there would be an equivalent to the Certificate Transparency system for keys that are used or protected by DNSSEC.