Generative AI is not going to allow media companies to not use VFX companies. They still have to employ someone to use generative AI (and other tools, it can't do everything yet), and that person is a VFX artist. The whole "It's just a tool" thing isn't a joke or a rhetorical device, it is literally true: this software needs to be used by someone who is trained in how to use it to get good results on a commercial level, and for the products we're talking about (Marvel movies) they are also going to need to be proficient in the rest of the VFX skillset as well. Generative AI replacing VFX workers entirely is a fantasy until the point where a company can hire/rent/buy/build a whole AI system (or company of such systems) which are plug-in replacements for human workers that are working remote. In case it's unclear what that means, that is human level intelligence. It then needs to be cheaper to pay for the computation that system requires than it would be to pay a human for an equivalent amount of work. I'm not saying that will never happen, it probably will, but it is definitely not the case with current generative AI tech and it's not on a reasonable five year time horizon.
The major impact generative AI tech will have on VFX is that because VFX workers who known how to use it well will produce significantly more or significantly higher quality output than those who don't, the bar is going to be raised significantly for how much work VFX houses are expected to produce. Given that, this is a pretty normal time to try and unionise, because with anticipated productivity windfalls the working class and the owning class are going to fight over who gets how much of those productivity windfalls. It's a possibility that less VFX workers get employed overall as a result of higher productivity, but I really doubt it. We have seen this story play out in VFX for decades: as tools get better and enable higher productivity, demand rises and more people are employed (still in shitty conditions though). You can also see it in the games industry, often with the same technology: did generative technology for tree asset production cause job losses in the games industry? No, it just dramatically increased the number of games (and films) that could have large numbers of realistic modelled trees, back in 2009. Since then demand for games and films has only increased and so have job numbers in both industries.
Current generative AI tech, when applied in VFX/games asset creation and post-production is basically a better version of SpeedTree for a wider variety of assets. It is likely to have similar economic effects to what SpeedTree did, which is to say higher productivity in cases where you can use it and higher achievement peaks in the the films and games produced. It shouldn't fix poor job conditions and it shouldn't make them worse in an objective sense. What will determine job conditions is union action or lack thereof, so it's good to see VFX workers doing their best to unionise right now.