I went a bit off track, but I did want to make a relevant point: For my use case, the features you enumerated aren't competitive advantages because they're lock

I went a bit off track, but I did want to make a relevant point: For my use case, the features you enumerated aren't competitive advantages because they're locked away in a single client on a single machine. Thus, for me, there are entire classes of features where native clients can't innovate, because I can't rely on those features being available in more than one place. So from my perspective, Mail.app isn't "way behind" Thunderbird. Rather, they're at parity, and Thunderbird has a lot of other stuff that I can't use.