This makes no sense to me. Tech workers have way more flexibility than many other types of workers. If you aren't comfortable with your employers practices, go find another employer. No need for a union to do that.
The owning class right now is collectively moving to worsen work conditions by colluding in getting people back into the office. Workers should answer collectively as well or it will not go anywhere. Sure, you can change employers, but more and more employers are cracking down in the same way.
Your solution of voting with feet is an extreme countermeasure, many people want to improve 1 or 2 things about their workplace, not leave. Unions help with all of this.