Usually pretty and efficient doesn't go well together. Pretty tools not only add useless things (such as animation, transparency, etc) that are not functional but consume resources, but also are designed towards looking good without thinking at the usability of the tool, for example a lot of whitespace and padding, big line heights and fonts, remove features that most of the user don't need, etc.
A professional tool doesn't need to be pretty. If you go to a plant control room and look at the computer screens they have an interface that looks Windows 95 usually... but that is fine, since they need to be functional, not pretty, they don't need rounded corners, they use high contrast between colours that are ugly from a design point of view but allow to see things easily.
Most email clients are shit. They show you mails in a conversational way that is just wrong, mail are not chats, but letters. They insist on composing HTML mails, instead of plain text ones (like Thunderbird does). And have a very bad UX in general.
Of many things that can improve Thunderbird (for example a better integration with Exchange/Office365 with not only the mail but also the calendar/contacts, sync settings to a Mozilla account, better search in the emails) they focus on the only thing that make most people use Thunderbird.
> I don't see the point of not wanting improvements
I see it. I'm used to a tool, that I use with satisfaction since years. There is no reason why I have to learn to use a new UI or change the way I work because somebody at Mozilla decided that we need a more fancy looking UI. The reason I choose Thunderbird is because other software, such as Outlook, became shit because they followed the same path of modernization that now is following Mozilla.
If something is not broken and users are satisfied with it, why the hell do you want to change it???
By the way, so far I'm happy with the improvements in Thunderbird that Mozilla had done, because having prettier icons and fonts, having the dark theme, are all UI improvements that are purely cosmetics and doesn't change the UX, same shortcuts that I'm used, same mode of operation, I don't have to learn a new thing. Now they decide that the whole UI must be modernized, I don't get it. I will probably stay with the last Thunderbird that will support the current UI for a long time...