This is true, but sort of specious. The point is that for the grandparent (and me) Thunderbird meets all these requirements, and other stuff doesn't. Could we adjust our decades-mature workflow to accomodate other tools? Sure. But it would suck. So we use the tool we have that does what we want until something unambiguously better comes along.

Of course! But Thunderbird does a pretty good job of all these requirements, it's the only reason I feel like I can be so stringent with them. Only reason I've been trying to leave is everyone else telling me it's bad or it's going to stop being maintained properly.