> If the best we can do is Thunderbird
What about KMail/Kontact? Evolution? Sylpheed Claws? The Client of the Vivaldi(?) Browser? Not to forgotten all the commercial clients. Are they all dead or trash?
> a slow-moving almost abandonware monolith (have a look at its extension store. It's a ghost town)
To be fair, Thunderbird has a complicated history. The unloved child of Mozilla, survived far too long on it's own, until it got love again, at the time when the parent moved away from XUL, giving the future of Thunderbird-extensions a timelimit. That it's still surviving on high levels is more of a miracle.