While he is pretty toxic, he is still right (it doesn't justify his toxicity though). You're going on many posts on the Boards about how it's a colossal nerf, but I don't see much information to back up your claim.
Miss Fortune isn't weak, against the misconception that started out from the Community and due to her lack of popularity, she is pretty strong and according to the matrix and pretty much all sites, she does more than what the average ADCs are currently doing. While I wouldn't say winrates in general are ever reliable, she has maintained almost a 52% winrate since her last tweaks a very long time ago and it rarely changes besides like, 0.01%.
Miss Fortune is an interesting ADC, her early game is probably the strongest, especially now in the PBE due to her Q reset+damage being essentially three auto attacks in one + her W's passive and Attack Speed. However one thing I see people missing is how powerful her mid game is. She's an amazing teamfighter and even stronger, she can hit the enemy backline and totally ignore tanks, her slow provides good utility, and while her passive was pretty useless in teamfights it's going to be much more efficient for repositioning or pursuing/escaping.
Miss Fortune can outduel most ADCs, not all of them. Mostly ones like Vayne, Twitch, or Draven who are more so designed as ADCs who blow up a single target rather than teamfighters (aside from Twitch an Draven's ultimates). Miss Fortune also has better teamfighting than most ADCs, but again not all of them. This is due to her ultimate being able to hit backlines and her Double Up as well. She can hit tanks pretty hard since she does a lot of Physical and Magical damage and is of course an ADC, and she can blow up squishies fairly fast. While she isn't the best in teamfighting nor dueling, she is strong in both. I like this, there are ADCs who clearly outclass her in certain categories but she has a powerful mix of both dueling and teamfighting.
Her main issues are her lack of mobility and fall off late game. Not her offense, which she excels at in the ADC category, and not her defense where she actually is a bit more tanky than most ADCs I believe. However her issues are not errors made by Riot, they exist because she NEEDS weaknesses like ALL champions. If Miss Fortune's late game was anywhere near as strong as her early game, she would simply be overpowered, and if all of her abilities could be casted while moving without her passive ever breaking off, she would again be overpowered. Imagine trying to run away from a Miss Fortune if her Q, E, and R had no cast times and her passive couldn't be broken. It would be ridiculous and unfair.
All-in-all, the removal of Miss Fortune's Grevious Wounds won't hurt her in compensation to what she's getting. It doesn't define her, and while it can make her unique in some eye's, that has no relevance to her strength. It's a 2 second Grevious Wounds in exchange for noticeable buffs on her entire kit. She's a strong ADC, anyone who thinks otherwise can look at me. I'm a Silver, casual, scrub who can play against people far beyond my elo, and still go something like 20-3 with Miss Fortune. I'm not a god at the game, but I've mastered her to the point where I can carry games like this with Miss Fortune as easily if I would be playing someone like Vayne, Draven, Caitlyn, Ashe, etc, etc, etc. She's a strong champion, and there's relatively no evidence against that, or I've yet to see it, and she will just be made even stronger with her new buffs.