If they're paid higher than average, then how could a union fight to get them higher wages when they aren't doing it for the unionized competitors that have lower?

And also per another comment here, they require an unblemished driving record. So if the worst concern of unions came true, that they can make it hard to fire bad performers and require hiring "their guys", then drivers with blemished records can make it in and stick around. That means the good drivers currently employed may see their value drop comparatively and likewise their ability to demand better pay.

As it stands it seems like the current drivers then have the benefit of being considered best in industry and may prefer to gate-keep and not unionize as a result in order to protect themselves.

I wonder if other companies that want to fight unionization should do similar: hire top performers and pay them well :)

I mean this also applies to the rest of Walmart, or at least it did

In high schoool and college (98 through early 00s) at age 16 I initially worked at a local grocery store chain that was unionized making minimum wage. They went out on strike for a month and came back to just a ten cent raise. Not long after that I went to work at Walmart making about 40% more, with a 10% discount on anything I wanted to buy in the store

Working conditions between the two were not much different, both were pretty transparent on scheduling, job duties, and the like

Now, I'm pretty left wing and I support the right to organize without any restriction, but I probably would've voted against a union if the UFCW had come to my Walmart store asking for signatures based on my prior experience with them

Does that mean unions have a place? Absolutely. We would not have the meager protections we have today without unions, and they serve a deterrent against employers running amok, but yeah treating your employees somewhat decently is plenty to keep them at bay