I can appreciate your assessment to a point. I no longer buy the theory that it depends on the line or strain. Over 68 years I've been around a few breeds and there are breed characteristics that are what they are. Temperaments can be slightly adjusted through selective breeding, plus other factors like space, mixture of breeds in the same flock, human handling, etc, and there will always be that one individual fluke chicken that doesn't fit the breed mold. But there are characteristics that are unmistakenly set in stone. In my experience there are the combative breeds, non-combative breeds, and some that fit in-between and can go either way. Case in point are the BRs and Doms which we've also had both. BRs are a branch off of Doms and BRs have been cross-bred with game birds, possibly Malay, and other breeds to make them more dual-purpose where the Doms have been untouched for the most part since Colonial times. The Doms are mellower than the off-shoot BRs. It's 50/50 w/BRs if the bird inherits its Dom roots or is a less gentle temperament like it's game bird ancestry. I love Leghorns but cannot keep these assertive birds in a flock of gentles breeds like Silkies, Ameraucanas, Faverolles, Polish, Houdan, etc. I speak of hen flocks as our experience with roos is extremely limited as I understand flock dynamics are different containing roos. Breeds crossed with Legs can inherit the Leg temperament characteristics and I stay away from getting breeds that have Leg history in them just to save grief. Pretty much can't go wrong with assessing the following breeds as known gentles like Ameraucana, Araucana, Brahma, Breda, Cochin, Dominique, Dorking, Easter Egger, Faverolles, Houdan, Pavlovskaja, Polish, Russian Orloff, Silkie, some Sussex, etc. But one can usually be certain of the assertive active wilder temperament of the following breeds like Fayoumi, Malay, Cornish, Gull or Braekel breeds, Marans, Mediterraneans (Legs, Andalusian, Ancona, Minorca, WFBS, Penes/Empies) and many more.
Dual purpose heritage or layer breeds are great for production and newbies should do research before settling on one of these breeds as they are not very compatible with the reputed gentles breeds - these heritage breeds are RIR, BR, Legs (or Mediterraneans), NHRs, Orps, Lorps, Wyans, Marans, and I would hesitate mixing large 9-lb breeds with 5-lb breeds as the temptation to bully will be there - it's a chicken thing. Nuff said.