The problem is that geometry references to a Rhino 7 file will be unavailable to R6 users, making it impossible for many people to work on R7 models posted to the forum or shared between colleagues.
If the geometry is not specific to R7 (like SubD), people can “File | Save As…” to Rhino 6 format but requesting that will get old very fast.
It would be wonderful for all concerned to have a R7==>>R6 conversion utility so R6 users can read R7 Rhino files, minus the uniquely R7 bits of course, which is what "Save As R6” does. Even better would be to have that conversion utility built into R6 and modify this dialog to offer the option to use it, with suitable warnings that R7 bits will be ignored, of course.
This approach would be WAY BETTER for everyone, allowing a smooth transition where people eventually upgrade to R7 because they want the new features instead of being forced to upgrade or be left out of the forum “party”, as was the case with migrating from R5 to R6 for nothing more than trivial changes to a few key components.