Well, one reason the analogy is bad is the HVAC industry does not have the luxury of copyright in the first place. They have to rely on patents and trade secret

Well, one reason the analogy is bad is the HVAC industry does not have the luxury of copyright in the first place. They have to rely on patents and trade secrets. The whole GPL concept is hinged on and is a hack on copyright and would be rendered moot absent copyright. As a practical matter, it's hard to come by software that comes as a freely licensed binary but lacks source code. I'm blanking out on coming up with even a single example. Re the ability of "study and modify" it does not have to be universal obviously. If a subset of community can do so the rest can benefit by their improvements passively, asking their help or contracting them.