I'll concede that DNSSEC is not in a good spot these days, but I don't know if that's really due to its design or lack of adoption (it's in similar territory as IPv6 TBH). DoH is (IMO) a poor workaround instead of "fixing" DNSSEC, but it's unfortunately the best way to get secure resolution today.
Putting aside the DNSSEC issues, IMO, DNS should be authoritative for everything. It's perfectly decentralized, and by purchasing a domain you prove ownership of it and shouldn't then need to work within more centralized services like Lets Encrypt/ACME to get a certificate (which seems to becoming more and more required for a web presence). A domain name and a routable IP should be all you need to host services/prove to users that domain.com is yours, and it's something I think we've lost sight of.
Yes, DANE can create security issues, your webmail example is a perfectly valid one. In those situations, you either accept the risk or use a different domain. Not allowing the behavior because of footguns never ends well with technology, and if you're smart enough to use .internal you should understand the risks of doing so.
Basically, we should let adults be adults on the internet and stop creating more centralization in the name of security, IMO.