There was probably a time when unions made more sense. When much of the work is exactly the same and people are just cogs in a big machine, well, I think the idea of collective bargaining makes sense. But while Amazon warehouse workers and delivery teams are doing pretty similar work, there are still differences. Some people just do more work and I don't see anything wrong with rewarding them.
Now all of that being said, I can see a better argument for breaking up Amazon to ensure more competition. I feel like an open labor market is the best way for workers to earn good treatment. When the market was tight, Amazon responded with higher pay and more benefits-- without the need of a union to get in the way and collect dues.