I've taken a look at my Rivens post-update, and my arca plasmor Riven just doesn't compete with normal mods anymore. Now, I'm going to have to re-forma my Riven slot so I can equip a conventional mod.
What is the use of a Riven for popular weapons now? Just take those weapons out of the Riven rotation, and owned the fact that Riven are for less popular/broken weapons. Getting an Arca Plasmor Riven now is like "I would have preferred an Ayatan statue: I don't have to unveil that before I turn it into endo, and I get more for it too!"
At some point, lowering dispositions just makes Riven useless junk. It's simple arithmetics. You should put a lower limit, or you'll just end up breaking the Riven system even more.
And one more point: I'll keep using my arca plasmor without Riven, and my ignis wraith (never had a riven for that one). And many other players will do too. So those 2 weapons will keep being on top of the popularity list. Does this means their Riven disposition will keep being lowered? Will they get a negative Riven disposition eventually? Or will you just go out of your way to nerf them, like you did so many weapons/abilities that players loved in the past (Tonkor, Synoid Simulor, Castanas+Trinity, Ember's world on fire, etc.)?