Firstly I love the game and all of that stuff... I typically like the way the dev team handles balance issues... The coptering / parkour system is a great example of that, etc etc etc...
One thing I'd like to note, balance passes with nerfs are cool in most types of games. In looters however, it's a slap in the face to your player base. If someone spends a year farming for the perfect gun/sword/riven/whatever and the dev team figures out they screwed up and couldn't figure out how to balance the game the first time, and then nerf it... You effectively just wasted that persons year. I personally find this to be completely disrespectful of peoples time in the best case, and in the worst case it's completely disrespectful of their wallet.
I have seriously stopped playing Blizzard games entirely because of the way they handled balances in Diablo 3 (Don't even get me started). Not just a game, the entire studio. At least at some point they realized that a real money market in a looter game with the way it was handled was BS (though it took a year or two). DE isn't close to that level of frustration (for me personally) but these things can, and do happen.
I mean... it's one thing if a mechanic is just released and there hasn't been a lot of time on it. Sure then, nerf and balance away! It's something completely different with the system you have here though. This is a system that's been in place well over the year I've been playing, and probably even longer! That being said, sometimes -- like with the nuke trinity build -- things slip through, I get that, and every time you guys screw up on game balance I'm not expecting that you completely re-balance the rest of the game based upon that.
I highly suggest that you just factor the riven disposition into the weapons damage potential and use that to determine MR (or figure out some means to statically allocate dispositions). For example if you want a Vectis Prime equivalent weapon with a high dispo just make it MR 18 or something. If you have 2 weapons at MR 18 and wonder why those are the only 2 weapons used.... If people what to roll a riven 300 times to get it right let them do it! I mean out of the 350+ weapons in the game you should be able to create enough diversity at the end game.
Conversely, I happen to notice that the way you are doing this (changing dispos after a new prime is announced/released) basically creates a lot of churn in the riven market to where new weapons will get inflated prices for awhile. I'm not saying the company is doing this on purpose because it affects plat sales or anything, but they should realize that this happens and adjust dispos well before there is market insight into the upcoming gear.
Sorry for the long overly negative post. I just needed to vent a bit. I'll promptly go off now and buy my 6th Prime access in a row... and who knows... I may pick up a new riven 😛 For the record, I don't buy prime access because I actually need anything in game any longer. I'm MR 25 and I have most every item in the game already (including rivens for every gun I like). I do it to support this company because I'm generally appreciative of the game and the community.
I hope the team figures out how to not have rivens universally hated for various reasons.