Re: "but they tend to retain support for legacy stuff"
Actually, until now Microsoft retained support for "legacy stuff" far more than open source.
The issue with Microsoft was the other way around. By periodically updating the format it meant your "legacy" version of Office couldn't open the .doc or whatever file someone sent from their more recent version so you had to buy the new as well.
The open approach is to set the file format as a standard and stick to it so our old version of the S/W will open and use a file written by the latest version or even from some other application using the same standard. Microsoft got its arm twisted to use an open standard so they got their own, reportedly of dubious clarity.