The IETF draft itself says that the primary motivation for MTA-STS is to provide transport security when DNSSEC is "undesirable or impractical". You don't have to take my word for it.

There are a million DANE-enabled DNSSEC domains because there are registrars, particularly in Europe, that enable it automatically. Who cares? First of all, DNSSEC managed by your registrar is security theater, but, more importantly, the overwhelming majority of those domains do not matter. Who cares if some landing page in the Netherlands has TLSA records?

Meanwhile, the domains that really do matter --- the ones managed by the major mail providers --- are doing MTA-STS.

SMTP is not a success case for DNSSEC.