"It's embarrassing that MS can't provide 100% backwards compatibility for their own proprietary format." My reason to get [#Libreoffice thread.]
Not that simple.
A #programmer, especially at a big name house costs money, $150k or more, not including bonuses. (I just retired from 39 years #programming at an indie shop.) Maintaining backward compatibility? it's more code to test and—if it fails or causes other #code to fail—debug. It's more code to write again when management changes, changes the programming paradigm, language requirements, storage access, privacy standards, auditablity, or who knows what. It requires at least a dedicated programmer across the supported products, maybe a seasonal team when code changes, and for sure a slice of management time.
100% backward compatibility for what? The vast number of users have no files older than a few years, unlike packrats like me. Max 7 years is a best practice in business. People actually update old files or PDF and archive them. Problem solved.
Moreover... there's always people who work for respect and no money that will do this scut work themselves, maybe because they need to open old files or simply that they can, and make it available for free and nice comments. The LibreOffice dev team, if there is one, sees it as a feature to increase the install base. Remember that they get donations so it's not exactly work for free. If this external programming fails, #Microsoft is more than willing for these people to do tech support, or let them get sued or boycotted—and suck up the bad PR.