They shouldn't "stop signing up". They should unionize. Many adjuncts are already doing that and it's helping.
For rough numbers, an epically good adjunct union might secure a 50% pay raise. But sane PhD pay rates would have to be something like 2x what they are now for it to be a viable career path for high-IQ white collar professionals. And that requires the supply to (take an economic hint and) contract.
Sure, there's massive unemployment. The same will happen with adjuncts if they unionize. But it'll be better than the current situation.
- Actors (in the aggregate) get a bigger fraction of the returns than the typical worker in other industries.
- The overwhelming number of people who make some money from acting have to supplement it with something else since they can only get a few days per year.
- Full time acting is not a viable career option for most people who would like to do it and are qualified.
- Unionization did nothing for the above dynamic, though it did have other benefits, like improving pay somewhat for those who find part-time work at all.
IOW, yes, it has benefits, no it will not return us to the state where getting a PhD becomes a viable full-time career path.