I'm not talking about a generic "unions are good" argument. I know why unions exist, the history behind them throughout much of the West, their ties to worker parties and socialism etc. I'm specifically wondering what about unionizing (or in this case, re-unionizing) makes it more palatable for these highly specialized roles to exist. Is your argument then that working conditions for folks like HVAC techs are poor enough that unions are necessary to negotiate for better working conditions before people want to become HVAC techs?

Generally, the more specialized labor is, the higher wage and benefits they can demand in a capitalist labor market. Supply and demand. For less skilled labor like general electricians I can understand the role of the union in negotiating for wages, benefits, and safety standards. For highly skilled labor like an HVAC tech, I don't see it.