Let's try to imagine a situation when Win10+ UI has "the same display-real-estate efficiency of Windows 7".
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.
I've been using Thunderbird since I don't even remember when and I never noticed that it's got a bad interface, unless when they broke manually sorting folders and fixed it years later. It's got the UI that a mail client has to have. GMail does have a bad UI. Sometimes I don't understand where a message ends and where a message starts. Luckily I don't have to use it.