So actually, you are harmed by this decision every day you use a Windows 10/11 laptop. You live with this every day. You just don't know what you're missing.
But even if I accept your premise, I’ll remind you that Windows 7 came out 19 years ago. I didn’t exactly own a 27” 4K monitor in 2009, and a 14” or 16” screen laptop in 2009 would not have been thin and light like it is today. 2009 was like “my bezels are over a half inch wide” era.
The OS is fully scalable as well. You can just make it smaller in a very granular way. Windows 7 wasn’t capable of that.
Would it change anyhthing? No. Because developers of the programs what are not part of Windows aren't keen on keeping "the same efficiency". Audacity 4, Thunderbird are good examples of a bad interface - and they did it disregarding the OS UI.
They are basically throwing away the existing UI asking with its decades of refinement in favor of something that looks like what you'd get if apple mail and outlook had a disfugured child.
That's because you are just accustomed to it [0]. If you would have a fresh look on it, especially on a small screen - it would find it atrocious.
There is still a separate SMTP configuration and it's a separate separate SMTP configuration for the each account.
There is absolutely useless left vertical bar which... does nothing what can't be shoved unto the useless, empty top bar. There is useless empty top bar which holds a search box and a hamburger menu.
For the unknown reason[1] this top bar is twice the tall of the regular menu. Oh. There is TWO search boxes[2]. Enabling Quick Filter adds another toolbar instead of using the bar where the button 'Quick Filter' resides.
And if you have multiple accounts and for some reason you can't auth for a moment (eg server problem, not with a connection) you would be spammed with the auth boxes and even if you successfully auth in one - you would still need to cancel or auth in all other what was spammed on you.
There is a lot, a lot of things where TB sucks. You just familiar with it.
[0] and probably configured it years ago and never touched the config too.
[1] actually it to be tall enough to hold hamburger button - because there is an option to add buttons to it. But you can't move buttons from the left useless vertical bar to the top bar. So you can have 'Mail', 'Calendar' etc duplicated on the top bar... for what?
NB yes you can hide the left vertical bar. Hide, not remove. Still it was thought on, implemented and shoved on our asses for absolutely no reason..