I guess the difference between grade 4+ and 5- is the consequences of messing up, the actual technical difficulty is probably quite similar.
If something is being run more and more often, then it isn't really a grade 6 is it. The river can just be downgraded - much the same way in which they are often are in low water.
It may still be grade 6 at certain water levels, so is it better that it is graded 5, encouraging more people to paddle it, or stays as grade 6? The suggestion that top end grade 5 is renamed grade 6 & that "unrunnable, except in exceptional circumstances" becomes grade 7, is basically doing just that. Of course a 7th grade already exists in the form of X or P(ortage).
Unfortunately our climate means rivers are often paddled at less than optimum levels, a good example of why expanding the grading system is fraught with difficulty is the Etive, at low water the sections between drops are grade 2, but how do you grade the drops? Grade 4 - because you can't see what is at the bottom - once you know the line they are mostly grade 3. However, with a decent flow it's definitely grade 4 & at high water it's grade 5. The grades of rivers are defined for paddling rivers at a "good" water level, not low water. From river description on this website:
Frazer Pearce offers an alternative view on the Etive....'The Etive is a ditch. Often paddled at ridiculously low levels by masses of paddlers who would be much better off walking in the surrounding hills. Worth paddling once in your life, but only for the photos to impress your mum. Most of the drops are massively overgraded, except for the portage on Crack of Dawn. Nothing else rates anything more than a 3 at the levels the Etive is normally paddled at. The paddlers who get the most out of the Etive have usually only been paddling for a few weeks, certainly less than a year and haven't run big drops before. Not much technical skill required unless things are honking in which case the whole thing is a different kettle of fish. Hint: you aren't supposed to be able to jump over a river.'