>>vimes6+(OP)
When I was in the VFX world we had fixed, hard deadlines. Once the release date is printed on a movie poster it's not changing. Some parts of the process were easy to estimate since they'd been done many times before, but on every film there were some experimental new things. Often they would just end up with 2 or 3 different teams working on the same problem. One with a known technique, and one or two trying something new. Then if the new stuff didn't work or was taking too long, you could always fall back to the old way. Which wouldn't look as good or achieve the effect you wanted, but :shrug:. There was always the chance that you'd work sometimes for years though and your work would get dropped which was never that fun for the artists involved. Most development teams don't have the resources to have an A and B team working on the same problem but it's a way to contain risk.
The studios use release dates as deadlines basically to contain costs. Otherwise some directors will just keep working on a movie forever (or keep going back to it like George Lucas). Some movies, especially with VFX, are never really done, they just get to the deadline and finish with what they have at that point.