I generally agree that Crimea is Ukrainian by historical chance. Although it is then as much Russian as it is Turkish, or Greek.
The legal system of Europe does not apply retroactively to the historical processes that had shaped the modern borders. The idea is, however, that the present borders should be kept inviolate regardless how they got that way.
But that is quite ahistorical.
The borders in Europe have been subject to constant change. There is no sovereign authority with either the power or the right to fix them at a given point in time, to stand "forever".
That the borders of today will resemble the borders in 50 years is quite unlikely.