Considering the plethora of options, I'd say it's impossible to say what is better until an alternative is tried. And then you can only say that particular alternative is not better than basic, but you still can't say basic is best.
People that style their apps try many alternatives, and often find things that work better than basic for them.
Sometimes it can also drive me to switch to a different app, like with Firefox. FF used to be my secondary browser, but Zen (a Firefox fork) aligns with my needs and preferences better and doesn’t require userChrome mods and addons that are likely to break after some random update some day, so I switched.
Thunderbird would benefit from its own Zen-like fork in my opinion. Its UI has always felt clunky and awkward, and the “new” design just shifts around the awkwardness.
On desk-bound machines hooked to 27” displays, this isn’t really necessary, but the UI being built around vertical tabs as the standard (as opposed to most browsers, where vertical tabs are a tacked-on afterthought if they’re even supported without addons) is still a relevant selling point.
yes, I'm not wasting my time customizing something unless I use it frequently.
Not a Thunderbird user, but the Outlook default looks similar to the screenshot on the linked page. Initial things that drive me crazy; 1) left pane is a complete waste of screen real estate. I have mine collapsed to just be icons, it's about 1/6th the width as what's shown. It expands if I need it to (on tap/hover). 2) I like my inbox above my message preview not next to it. On the inbox pane, I get From & Subject on line 1 and initial message text on line 2. Same real estate with more content and context. I really like having the message preview line without actually clicking on the message.
Also, by having the message preview pane wider than tall, long paragraphs do not wrap so abruptly and I get more content on the screen. This lessens my need to scroll unless the message has a lot of paragraphs or images. Same for the initial message preview that's visible in the inbox line 2, if it's wider I can see more text. For a lot of emails, I find they are short enough that I can read it all in the inbox without even looking at the message pane. This means I can scroll/scan my inbox quickly without opening each item in the message pane to view it.
Anyways, I wouldn't care if I didn't use Outlook daily. For some people, maybe the defaults work but I feel like I get a lot of productivity out of these minor customizations
I still have Thunderbird installed and configured for the odd "advanced task", but my daily driver has been Geary for over 10 years now. Precisely because its so much better looking than thunderbird.
Looks, done properly, are an important part of a good UX. For me, less clutter, more whitespace, clearer focus, works far better than the "widgets thrown onto a square" that Thunderbird is (I have ADHD, it may contribute to this preference for clean, simple, focused design)