But in the context of a web app though? That's the key part: you're taking in email addresses from myapp.example.com. For other cases: sure, local email addresses can be useful (not often these days, but sometimes still are). But in this context I'm not really seeing any use case.

At previous $work, I wrote a rails app that managed releases. On the app you could sign up for email notifications when your change was released. These went through the unix local email system. So yes there is very much a use case

It's still a bit silly to tell every SaaS developer that they shouldn't validate that they have a normal-looking domain because some people's use cases call for sending emails to local addresses or to TLDs directly. If a developer needs to handle those cases, they know already, and they're in a vanishingly small minority.