Brad Plumer makes the point that laws favoring unions might depend on union strength, rather than union strength depending on supportive laws. Proponents of favorable union laws tend to believe they will revitalize the unions. However, these laws may not even come into being unless unions have a strong presence to begin with.
Do private-sector unions still have a future in the U.S.?
I see this as somewhat analogous to the work safety and OSHA laws. It seems to me that OSHA laws were the effect of increasing worker safety, not the cause. This is quite noticeable when looking at a chart of fatalities per 100k workers.
This chart doesn't prove that OSHA is useless, but it does clearly show that these trends were already in place for a long time. It seems unlikely that OSHA and other similar laws are even passable unless they are already essentially standardized. In the future OSHA laws may assume zero deaths of any kind, but only because advances in industry have made that feasible.
To assume that OSHA and similar regulations are responsible of increased workplace safety is a bit like putting the cart before the donkey, and union regulations fall into the same category in my mind. In fact, I think this mistaken source of causation probably happens quite often. People see good results from some law and assume that the positive outcomes that followed were the result of, instead of incidental to and possibly causal to, those laws. Correlation does not equal causation as they say.