its mostly dying because its so poorly optimized it chokes on mailboxes that have more than hundred emails and cant do search properly. One of the worst FOSS out there.
Hacker News users are much more likely to have vast email archives organized into folders which must be regularly searched than the general public. Most people have no choice regarding work email. Web email clients and the default mobile clients meet most people's personal email requirements. In other words 3rd party email clients are dying because there is a lack of demand.
The search functionality definitely could use work. But that’s my point. What other desktop email clients are you going to use instead?
Not GUI per se but there's mutt that has high performance. That's why GUI clients are not popular anymore either, because they perform very, very poorly.
I have mailboxes with tens of thousands of emails in them, and it works pretty well for me.
Try a search and be in awe at how long it takes to spit results.
Are we talking about Thunderbird because it performs very well.