The result is that leaders at the district and school level tiptoe around us, afraid of angering union leaders. I've never felt so respected and cared for as a teacher. I would never go back into a situation like you describe where admin has free rein and no obligation to show appreciation or concern for teacher workloads. With the union, I can truly focus on doing what is reasonable and effective knowing that I'm not going to get extra tasks or extra students dumped on me without compensation.
And no, they don't have the option of replacing me for someone more gullible/desperate because they are forced to treat everyone well. What actually happens is that the macho/ego types don't go after the admin positions because those positions are not all-powerful; they are positions of service to teachers and students, as they should be.
Nothing is ideal. Sometimes the union goes too far. Sometimes teachers demand too much leeway and it does affect teacher reputations negatively, but it's overall much better for creating meaningful jobs with a limited number of tasks that can be executed effectively.