"Negotiating on your own terms" is definitely a wild thing to see as a positive. You may believe you have some special bargaining power as an individual but as proven by history and some basic power analysis your ability to better your position in life is absolutely more effective through a union. "Negotiating on your own terms" is fine if all the bosses find your personality pleasant and your sociocultural background comfortable. What happens is that talented people don't get their just rewards from hard work, educational background and talent because the bosses believe they can get away with it and know that the employee doesn't have much to bargain with on their own.

And the people "who should've been fired long ago" are also people who have lives and families and other obligations that make protections against arbitrarily firing someone a social good. Because unions force employers to fire people on the basis of quality of work and conduct (or economic concerns) rather than arbitrary personal preference and bias, your hard work is more likely to be noticed.

I live in one of the most heavily unionized places on earth in Scandinavia and have worked on both sides, as manager and as an employee, and if someone causes a legitimate problem, you can 100% fire them. You just have to do your due diligence and prove your case about the worker in question. The only thing is, if they have worked for a long time and you need to do a round of layoffs, the people who have worked the longest will not be the first to be fired. This helps protect against companies using layoffs to fire expensive employees and replacing them with cheaper newer hires.

In short, "individual negotiation" is basically saying the boss gets to decide.