Even if this we take this for granted, its completely untrue that all unions commoditize a trade by flattening pay. There are plenty of unions that have chosen not to put standardized salaries in contracts or have advocated tiered salary agreements or merit pay. Whether or not these are good things, they are things that unions have done and should dispel with the notion that unions are inflexible towards these concerns by workers.
As for your concern that unions "force employers to do things", well I suggest you consider all of the ways your employer can coerce you to do things and whether you actually have a mutualistic relationship. Many workers don't feel that they do.
...which hardly can be said to be acting in the best interests of either small-time musicians or big stars.
The RIAA actively seeks to withhold royalties from musicians and intentionally makes it very difficult to collect them. A number of big-name stars sued them over this some years back, unsuccessfully.
Unions were reinforced by the NLRA to enable workers to bargain collectively. That can mean a lot of things
A union for software engineers is a blank slate. It hasn't been done before. It could be anything someone makes of it.
The problem is that if I had stayed there for thirty years, I would have been pretty much guaranteed to make good money. By changing jobs a couple times, I made that same money in 2. So... people who are capable of getting better jobs and willing to risk change simply left, while people looking for stability or who had trouble getting jobs simply stayed. This didn't lead to the type of environment that I enjoyed working in. Your mileage may vary.