I'm not sure I agree with your assertion that people getting hired and fired within 12 months "ensures" you only work with the best. After all, if we zoom out for a bit, we'll notice that there is not a continual influx of fresh talent (across the spectrum), and likewise there really isn't a continual exodus either (of, say, the poor performers). Instead, I'd argue that in software it's more like a drip feed going into a dish. A little comes in, a little spills over the sides, but there's still a lot in the pool. Someone who is working in software is not going to exit software because they got canned in 12 months. They won't be like "oh, clearly I'm one of the poor performers, time to turn in my badge." Nope, they'll go to work at another software company. Maybe they get fired again, or maybe they're exactly what that company needs and are perceived as "the best." As the saying goes, "one man's trash is another man's treasure," so unequivocally saying you work with the best, or that only the best survive your workplace, or any workplace, is just asinine. As in most things there is a distribution of talent, and talent is a multifaceted thing. So "best" really is an imprecise way to put it. Perhaps where you work, the "best" which survive are all selected for their JavaScript coding skills. They may be douchebags to work with and for, but all that matters is the JS so that's what defines who stays and who goes.
I suppose I'm coming on strong here because I'm tired of the standard line of bull in San Francisco (especially) about how "we only hire the best and brightest, so if you're lucky enough to be invited to join the team you'll be one of the elite few." This mentality is what supports the myth of the unicorn (someone who is amazing at everything). I have worked with some amazing people, but none were "unicorns" because all had (as all humans do) their flaws. One of the brightest people I've ever worked with was also one of the biggest a--holes. It was very difficult for him to reign in his mouth. Was he the "best?" It depends on how you define it. But not to me. I don't care how brilliant you are if I can't stand to work with you.
Personally, I look for the sweet spot. My "best" is generally nice, good people who respect others, are resourceful and motivated to do good work, and who I can count on.
So anytime someone tells you they only hire the best and brightest, I recommend extreme caution. This likely means compensation will be lower (after all, you're being sold the "benefit" of only working with the best and brightest), and the environment is likely to be filled with lots of arrogant people who are oblivious to their own flaws. Ugh. No thanks.
So that turned into a tangent.
In spite of all that, I too don't support or like unions. As other commenters have stated, they end up catering to the lowest common denominator, and don't serve high performers (consciously using a very subjective descriptor here) well. They also may not even serve their members all that well as the union entity ultimately only cares about its own survival. Many years ago I interviewed for a union job as a package loader for UPS. I was surprised to find the wages offered were low, I would've been required to furnish my own equipment, etc. There was no observable benefit of union membership, yet if I had accepted the offer I'd have been required to pay dues. Furthermore, I had friends who worked for Delta as baggage handlers. Their experience was the same. Sure this is all anecdotal, but from where I'm sitting the only ones really helped by the union are the union leadership.