I'm not entirely opposed to something like this, but I don't like not being informed of the relative downsides of something before spending my time on having my CMDR acquire it. A chance for malfunction that only shows up at G5 would mandate many extant G5 modules be remade at a lower grade to serve there purpose.
I don't think FDev would ever for it without either grandfathering all extant stuff, or buffing G5 module's positive effects further. They have proven exceedingly adverse to most nerfs and the last the game needs is another wave of legacy modules and buffs.
I think any positive change is unlikely, but the most likely ones would have to apply to everything, so they at least kept their relative appeal to each other. Targeting only the top-grade seems overly arbitrary.
In my head you would be explicitly told about the malfunction risk- it would never be black boxed.
For me the problem is that G5 these days does not come with downsides and you have to have them. For example OC powerplants enable everything on ships, and can only be countered by direct damage that for most never happens. So its not a downside that might make you think to keep lower. There are vanishingly few cases where G5 is not everything.
It would be nice to be able to downgrade and reclaim mats though- sort of liquidation.
All nerfs are unpopular, so the best nerfs are massive ones that achieve the desired end result in one blow.
Weaning people off of current heights would result in more total and much more protracted ill-will than just kicking them in the nuts and being done with it.
I simply can't see FD doing that sadly. Plus I'd rather have the agency of risking it all on a crazy build that could fail (or not if you decide to).