> Employers already have most of the power in this relationship

I feel like it's the exact opposite.

I once was at a team where a coworker left. He was one of two developers working on a subsystem. The subsystem was highly valued by the company because it would result in faster payments and less labor in processing payments. Even though they replaced him with another developer the project still took 1.75x longer to complete compared to the previous pace.

When a developer replaces a company they continue to pay the same green money. When a company replaces a developer it can take months or years to retrain him with institutional knowledge. The asymmetry gives developers more power in the relationship.