I like the new UI way more than the old one. It doesn't look as cramped.

The lack of information density is generally what I hate about modern UI design.

This is the reason I use mutt to keep my inbox organized. It's a bit spartan for an all-around email client, but when it comes to archiving and deleting mail, it's so much faster than any graphical client I've used. 50+ emails at a time on the screen and nothing else, just hitting a key to tag them and then committing the delete is great.

I get a LOT of mail. The information density in mutt allows me to accomplish a feat of organization that felt impossible for years in Thunderbird.

You can control the UI density. Mine is usually set to the most compact setting and I really like the new UI (my only gripe would be the big blue button which could do a lot more).

Preach! I'm using a computer, not a phone. I want to take advantage of the extra real estate and input precision.

I like the new UI look a bit more myself. But I get where you're coming from.