... I've begun to wonder if, with enough time reading such threads, one could become very, very 'authoritative' without ever actually making any photographs. Can you imagine the scenario? Just keep reading until you seem to know all sorts of things, develop opinions on them, and then offer those opinions with conviction. But never take a picture.
Could this happen?
Absolutely. But without some degree of lab work and professional guidance, most are limited by constraints of self-education.
For example, imagine studying to become a pharmacist. Anyone can pickup a PDR and a chemistry book and start down the path of learning pharmacology. One could become an expert without any formal training whosoever, and also without ever doing any lab work. But only a few would be successful using the non-traditional approach.
You could make about the same argument as above for someone who was self-educating themselves to become an attorney and also in the example starting this thread, a photographer.
The core component of the OP’s question revolves around what it takes to be considered an expert.
In the case of pharmacology or law, one is considered an expert when one can pass the qualifying tests (the bar exam or the pharmacological board certifying tests) in the examples given. These tests are set by the industry’s and the State’s criteria for determining expertise. In both cases, if the student can pass the tests, then they are qualified to do the work. Similarly, in photography, if one can demonstrate a wide variety of skills and expertise, then they are considered an expert. It doesn’t really matter how they got their training.
The gotcha in photography is that there is really only one way to demonstrate expertise...
In these 3 cases it is possible that a student of ability could become an expert with no formal training whatsoever. But it doesn’t happen often.