I'm with the engineers on this one........ we all know that "engineered" bridges never collapse, "engineered" high rise buildings never fall.......... I still don't know how those old barns were built without an engineer. Carry on...........
I know how most were built; and that was poorly. Most old barns have failed. The ones that you see today that haven't failed were the ones that for one reason or another were constructed better than others.
Some were built well, and there was some cultural learning from failed ones that improved other ones being built. But, without a systematic way of designing and improving construction, many failed.
Engineering is just a systematic and knowledgeable way with insuring that the design is adequate by calculating performance in accordance with sound materials properties and physics. Nothing more, nothing less. Design by "looks good enough", and advice on the internet from unqualified persons is the opposite of that. It's just guessing, without anything except that persons previous experience to back it up. Depending on the previous experience and the persons competence in interpreting that experience, the advice may be good, bad, or awful.
I do know that county agricultural structures here that do not require a building permit often fail. There were numerous failures here the last winter of ag buildings, I did not see any failures of engineered structures, although I am sure there could be some. The probability of failure is much less with an engineered structure.
To address your specifics: Engineered bridges can, and do fail. But they fail at a much smaller rate than bridges that are designed without engineering. High rise buildings can fail, but I can guarantee that 100% of high rise buildings would fail if they weren't engineered.
Not all construction needs engineered. Not all truss modifications would need to be engineered. But, they do need to be competently designed. One of the best ways to insure competent design, and to be fully supported in our code and judicial system is to have that design done by an engineer.