Funny I should see this today, I spent yesterday afternoon reading articles about Thunderbird and other mail clients and ended up migrating from Thunderbird to mu4e [0] (purely for reasons of usability).
If this is true (and there is some reason to doubt it, as this isn't the first time Thunderbird has been declared "dead"), and considering that Geary is no longer in active development either (maybe the Elementary OS guys picked it up?), Mailpile [1] might have a chance to fill that gap. Other than that, there's always Claws Mail [2].