Less jobs, mean same number of people fighting for same positions. It's hard to have leverage when people will do your job for less.

Fewer spots means more musical-chairs style competition and undercutting.

The existing pool of VFX artists wont just dissapear immediately, they'd still like to work in the fewer available jobs post-AI

The real reason is that VFX is not a major cost driver/bottleneck for movie production. If you make VFX cheaper but acting still takes up the same amount of time, you can't expect that more movies or series will be released to absorb the VFX staff. The real economy has a leontief production function.