All above average employees I know don't need to worry about staying on the better of average for some arbitrary metric. If the company makes a mistake and fires them, they can quickly get a job elsewhere. It's the people who have no better alternative than their current job who need the protection provided by a union contract, and they have no better alternative because the company made a mistake in the other direction when agreeing to employ them, and the union's job is to bar the employer from correcting that mistake.
And yes, unions successfully fought for improved work conditions 100+ years ago. What have they done to earn their keep during the careers of anyone posting on this site? Why should I as a non-unionized worker today join one? It's certainly not going to improve my total compensation, and the people who did the hard work you mentioned are long gone so it's no benefit to them for me to pay dues into an organization they were affiliated with at some point.