I think you might have gotten so caught up in the argument you’ve confused some of the details.
Scrivener for Windows 1.9 is the current released version of Scrivener for Windows. It generally writes out files compatible with Scrivener 2.x for Mac (the previous release).
Scrivener for Windows 2.9 is the current beta release of Scrivener for Windows 3.0, which, when released, will be intended to be compatible with Scriver 3.x for Mac. Because it is a beta release, not intended for production work, it only supports the 3.x format. It is NOT intended for every day use and being interoperable with the current released version, and by not including this code in the betas, L&L widely reduce the scope of potential bugs they have to deal with, some of which may have nothing to do with 3.x.
Scrivener for Mac 3.x is the current released version of Scrivener for Windows. When 3.0 was released, it included the “Save as 2.x” functionality. As I understand it, that functionality was included not to provide seamless integration between versions, but to keep the upgrade from being a one-way trapdoor and to allow people who use both Mac and Windows to continue to move files between platforms, albeit with loss of functionality when doing so.
We presume that when Scrivener for Windows 3.x is released, it will follow the same pattern as Scrivener 3.x for Mac in regards to this functionality – provided for convenience, to allow users to have a chance to work on projects in 3.x for a time and move back to 2.x if they need to. It will likely again involve some loss of functionality to step back.
Expecting ANY developer to provide forwards compatibility for non-trivial formats so that an older version can load documents created in a newer version with a different format is, well, a bit naïve. Presumably the new format was designed for a reason, and those reasons are tied to new features (or cleaned up codebases that are easier to test and debug). Putting the effort into allowing the old version to access the new format with full fidelity is giving the upgrade away for free – which, if that’s your business model, is great, but if it’s not no shame there either.
The only non-trivial example of providing forwards compatibility that I can remember was from Microsoft – the Office 2003 plugins for the Office 2007 XML-based formats. Even then, there were lots of issues with those plugins and they were meant as a temporary solution while an organization was upgrading to the new 2007 suite. There were pretty significant reasons to move to 2007 (the new formats were much more stable, not to mention significantly smaller on-disk in most cases). Even then, though, the plugins were a hack, not a solution. The solution was to upgrade.
At any rate, the biggest point of confusion I believe you have is that only L&L staff get to decide what they have to do. You and I and the rest of the customers can request but they are under no obligation to fulfill our requests. Which is good – customers frequently request contradictory things. KB has a vision for what Scrivener is and how it ought to work, and he is completely within his rights to say, “No, that request is not compatible with my vision.”