Comment by mschuster91

> Can modern Office reliably open the old formats? If so they must have implemented the parsers correctly no?

It can. But at two costs: first, MS has to keep all that legacy garbage code around - and we all know that just blindly deserializing stuff from potentially hostile input raw into C structs is ripe for getting exploited, and there have been a lot of bugs in there. And the second cost is, you can't reliably save an old document into a new XML document, hence the warning you get "this document was created in an old version of Office, are you sure you want to save it".