That has not been the case since WW2, and for good reason - defining national borders based on strength alone has caused two of the biggest wars the world has ever seen, and there has been a broad agreement that such devastation is not worth enduring for the sake of autistic map fetishes.
Also you are basically describing Russia there, not Ukraine. Russia seems to be opposed to Ukraine existing as a nation state, Putin himself has explicitly stated this in an essay last year and a speech last month; I have not seen any Ukrainian government officials saying Russia should not exist.
I agree Ukraine cannot rely on foreign protection, nor can it expect to defeat Russia in a conventional, let alone nuclear, war. But it can do what Finland did - make invasion and subsequent occupation extremely painful, and therefore politically not viable within Russia, and ultimately free itself.
It's been done countless times before in small countries occupied by great powers.