The world isn't so black and white. Document serialization is a rocky landscape that is rife with compromise. You have to balance document open time, document save time, file size, backwards compatibility, forwards compatibility, recovery modes, interoperability, size in memory, parsing time, time to save to disk, proprietary embedded file formats, metadata support, and more. And those are just the development considerations. You also have to think about upgrade cycle, time-to-market, third party integrations, and what will help you win marketshare and sell copies.
Software is hard. I think it's pragmatic for software vendors to have a strong, transparent philosophy about the trade-offs so that consumers can make the right choice. As the grandparent points out, Microsoft values backwards compatibility. If you value that too, buy Microsoft.