A few random thoughts…
There is a massive industry-wide labor shortage and a massive surplus of work to be done. Artist rates are way up, there are more junior artists than ever, there are more shows with more VFX than ever, VFX shops are turning away tons of work, new shops are springing up, investment groups are buying and consolidating VFX houses to try and become big players, etc. This is a boom time for VFX.
This is absolutely a (labor) seller’s market. Individuals and companies that are not taking advantage of this to create a better environment for themselves are missing a huge opportunity. There’s never been a better time to quit a job you don’t like and find a better one. Or to start a company!
It’s not actually true that the contracts allow studios like Marvel to make whatever changes they want at any time. I’m sure it can feel like that to artists at times but that’s not how it works. Contracts are negotiated at a fixed price for a certain scope of work, with the tacit understanding that both parties will be flexible. If requests for changes push that scope of work outside the original deal VFX studios can negotiate overages and schedule changes (and they’re free to attempt this at any time). It is totally within their power to be firm and demand livable contracts. But… it’s a subtle art. Like any human endeavor or any contract worth tens of millions of dollars trade offs and accommodations are made. No vendor wants to leave a client high and dry or totally screw them over, and no client wants to ruin their relationship with a vendor they rely on. I can’t imagine there’s anything about this that is specific to either the visual effects industry or the movie industry. There are a lot of people running VFX companies that are simply bad at this part of the job and get themselves into hot water. When that happens it’s the front line workers (as always) who have to work the extra hours.
That said, there are a number of (very large) companies where it is absolutely normal to work 6 days a week 10 to 12 hours a day. This is not a secret, it’s been very common for a very long time. Personally, I wouldn’t work for a company like that.
What we’re experiencing is a huge shift in how movies are made and an industry that is struggling to keep up. Workflows, infrastructures, and tool chains designed for how movies were made for decades are being pushed to the limit, both because of the volume of work and the way in which people want to work. We’re moving from a world where everything had to be planned way in advance (because of how massively expensive and slow everything was) to one where we’re expected to be more improvisational (because of how much much cheaper and faster it is to do the work). You can’t characterize that as a change from a good way of working to a bad one, it’s simply change. One with countless industries have also faced.
And keep in mind that just like any other industry, many of the changes to the film/tv business are technology driven, and VFX is at the forefront of that. So producers looking to totally reinvent how movies are made end up using the VFX industry to do it. If you take VFX out of the conversation there is little that’s changed (technologically) about how movies are made in the last 20 years. So VFX is baring the brunt of the changes that are happening in the larger industry.