Wuzzy, then why is UI not this now:
Because it is crap. Duh!

What a silly question!
But our current GUI is not crap. I think it is good.
I think the current GUI is good and works well. I definitely do not see the need to nuke the current GUI completely, this just seems insane.
Nuking the GUI needs SERIOUS justification. The reasons brought up so far were all far too weak.
Also, nuking the GUI is a serious investment with no guarantee that the new GUI is a major improvement. It's a risk of which I am sure the core devs are not easily willing to take, especially with such a weak justification.
Let me go through your points one-by-one:
- It's impossible to play on small screen, button sizes aren't uniform and hard to press on smaller screens (and misclicks are annoying)]
I am not familiar with STK on Android (does it exist?). Are small screens even targeted. Define “small screen”.
Let's assume you're right: This seems to justify a nuke at first glance. But maybe it would be more sensible to have a separate GUI for small screens. It might be too difficult to have a GUI which works both for small and big screens.
Also, we might want to hear more player voice here.
- Style of the game is different from style of interface, something that is quite natural in a game that has existed for more than 10 years
I don't really understand this one.
PLEASE be more specific about problems with the current GUI.
- Style of interface along isn't consistent, using elements of Tango as icons (which are way more detailed then the rest of icons)
Just fix the icons then, definitely not worth wasting a nuke. Do you know how expensive nukes are?

- UI is optimized for mouse and keyboard and is hard to navigate with controller
WTF?! I can absolutely not understand the criticism about controllers. I have played STK with controller extensively and I felt no inconvenience in the main menu at all. I can go through the main menu with only the controller just fine, thank you very much.
If you felt inconvenience with controller, again:
Please pinpoint SPECIFIC problems. Otherwise we are getting nowhere.
- On-screen keyboard is a mess
I have never used this, so I can't say much here.
This basically just sounds like a bug. Probably fixable without nuking.
Of course the current GUI
can be improved, just in smaller steps. I agree with Alayan. I do not say the current GUI is untouchable.