There is no broad support for unions not because of the bogeyman arguments your co-workers were raising but because for most US based software developers there is no acute problem they feel that unions would solve because almost all of us are doing pretty well. On the other hand no single developer really has that much leverage because the importance of even the best individual devs doesn't compare to an individual LeBron and/or there isn't the massive amount of money at stake that depends massively on the work of specific people. Unions for writers, actors, NBA players, etc are frequently representing for extremes of the totally destitute who are easily taken advantage of or groups of the ultra important where the Unions are essentially acting as high powered negotiators for those they represent. All very good reasons for unions, none of which is true for developers in the US right now.

I'm not saying there aren't things a union could help with, I'm just saying that for most devs, those things aren't pinching very hard. Also, you have to be realistic about the impact of unions, they are a double edged sword. Police unions aren't a straw man, they are a real example of the worst case scenario for abusive concentration of power and agendas at odds with the public good. The best examples always come with some bureaucratic overhead. So it is valid to consider the cost/benefit of unions. Again, I'm not taking a stance on what is right here, just saying unions are not as obviously a good idea in every case as you might imagine and the hesitance of your co-workers, right or wrong, is at least understandable.