I sort of understand what you mean. I have never wanted to go back to an older version (I myself have upgraded my perpetual license without Sphere), but it's been a mainstay of Studio One that newer versions with new features (even if there was a major update like 3.5 or 4.5) cannot save files directly compatible with older forms of the software. For some reasons, that is impossible to work around, for instance features like Multi Instruments, audio routing Splitter, and the Chord track simply do not exist in Studio One 2.4, so saving a backward compatible file would remove those features. An Aux channel could be converted to a normal audio track, but many other features simply do not exist in older versions, so undoubtedly newer versions saving incompatible formats is simpler for everyone to understand and eliminates the need for people to understand what features were introduced between which versions to know what will be erased when they convert down. Microsoft Office warns the user of this when they save an older format which is incapable of describing newer formatting options.
All that said, Studio One primarily works with information in XML format, so it's entirely possible to identify which features do not exist in older formats of .song files, and erase them, at the same time spoofing the version of Studio One the .song file was built with, so the file would actually load in older versions. The issue is, I'm not sure how often this actually comes up. Since I deal with perpetual licensing, I have all versions I own of Studio One installed, and should I need to go back to an old file that I don't want to upgrade to a v5 format, I just do that in the old version and avoid upconverting it. That in mind, I never want to do any of that, because the newer editing features from v3 to 3.5 to 4 to 5 are so much better that I would rather upconvert and keep working. I have never had to deal with others using older versions, because the new versions have thus far always been such improvements that if I did work with other S1 users, they're upgrading too.
Now, I am upvoting this because it is a decent premise for a feature request, and folks shouldn't be downvoting solely because it looks like a lament for user error. XML can be edited, and this is possible to do, just probably not yet a common problem for Studio One users--not that it couldn't become one, like the product segmentation Avid created with all those prohibitively expensive Pro Tools upgrades different people would wait different lengths of time to buy. I don't think PreSonus has that problem yet, but could encounter it if they aren't prolific in their market study and development.