That is mostly speculation built on speculation, but to address one more substantial point:

> ... speaking of which: the 3Delight engine itself has been in continuous development, latest release is from last year. But Daz3d couldn't be bothered to keep up it seems.

Any update beyond the version of 3Delight in DS would have stopped supporting RSL shaders, meaning every material would need updating and every sahder set would need rewriting. The change would have been as great as the change to Iray For usability purposes there wasn't an update for 3Delight and that is why Daz didn't add new versions.

I also have to point out that Daz did in fact release several new versions of both Bryce and Carrara, and has of course added many new features to Daz Studio (without needing to update the version number each time, one of the benefits of a free application).