Stagnant is a negative characteristic. Keeping things that work the way they are isn't.
MSPaint hasn't been "stagnant" for decades, it was the way people liked it.
I think one thing that maybe kept ThB from innovating was that people took webmail to heart. It's few people who want a thick client or even want to go thru the config. What port, what server, etc.
On a mobile device, the UI walks them through the scenarios (Google, Microsoft, etc) so they usually don't have to guess or have "IT" set it up for them.
Plus, what ships with MAC and Windows is good enough for most people who do want some kind of thick client.
Regardless, ThB was the ugly child at Mozilla since ever. They should have taken the Opp when Eudora was finally killed, but I don't think Moz had the energy.
Airmail and Spark, among others, would like to have a word with you. Airmail in particular started out as a Mac client before being ported to iOS. There exists quite a market for good email clients, I would think.
https://addons.mozilla.org/en-US/thunderbird/addon/gmail-con...
It resembles the old Gmail interface, and it's open source.
Thunderbird will soon add support for webextensions, who knows what will happen of the legacy extensions further than short term ?
It seems to be a totally different thing than the regular message list, and only some actions work in Conversation mode.
Thunderbird is the email client for people who want email to work exactly like it did ten years ago. that's their value-add, and it really should be their vision.