> However, certain employers at the margins try it first, then more prestigious ones...

A slippery slope argument. The fact that there exists a gradient of willingness to monitor remotely isn't a sufficient (or even necessary) condition to guarantee that most workers will be monitored: equilibriums can be established at 0% or 100% or anything in between. If remote monitoring isn't tolerated by individuals, I don't think it would become a prevalent practice, even without coordinated collective action.

Apply your logic to pay. Some employers pay very low wages. Others pay high wages. If you just quit your job, will the system reach an equilibrium with low wages? No, in fact, we know that wages in tech are quite high.