I co-founded a business, specifically around email testing, largely for the reason you mentioned. When developing we often skipped things like plain text, and usually only did a cursory glance over other aspects, very much less did we automate anything. It came back to bite us so many times (presumably countless times that we weren't alerted too also).

Obviously I'm entirely biased, but to your point on business-minds pushing for edge I'm even more convinced that manual testing alone is never going to cut it going forward.

Shameless plug at the bottom as it's rare that email testing gets limelight on HN :) https://mailosaur.com