When given evidence that it's at huge cost and investment that they manage to do that (basically an admission from them that it is hard — you may have a different definition of "hard" than I do), you seem to suggest that either they are not doing a lot of work towards compatibility or that they are wasting all that money because they could have simply maintained backwards ABI compatibility while developing new features "easy-peasy".

Why did they choose not to take the easy way out and made it hard for themselves?

From my software development experience, maintaining API backwards compatibility is "hard" (requires significant extra effort to achieve compared to just not doing it), not to mention ABI complexities on top.