Allow saving to previous version format
I upgraded from CC2017 to CC2018 (hoping it would fix some issues I've submitted elsewhere) not realizing that my co-workers who are still on CC2017 would then not be able to open my .indd files. Saving as .idml is a really, really suboptimal solution. I know other software I've had allows you to save in previous versions' formats; why can't you? Also, it would have been nice to know this would happen before I upgraded...
What's the point of all these new ID file formats? Didn't they already change it late 2016? Were there so many file level improvements to justify doing it so soon again? I can still open new Illustrator and Photoshop files across versions of those apps, why is InDesign treated differently?