Except in situations like a court hearing where someone is asked whether they are an expert in something and they say yes, most of the quality people I've run into would take a more modest approach and talk about the subject and let you decide whether you accept their opinion. It's also an acknowledgement that no matter how much you know about any topic there's usually even more to learn. So outright stating that you are are an expert is often posturing.

I think excessive modesty is kind of pointless. People who are looking for an expert in a thing often don't know what constitutes an expert in the thing in the first place. You should not have to recite your whole resume to be able to call yourself an expert. The point of having the word at all is to efficiently communicate an opinion about a person (even yourself). When does it become ok to call yourself an expert? 20 years? 5 books under your belt? The most authoritative book under your belt? What?

I get that there is always more to learn, but there's nothing wrong with being real with people about what you're good at. Maybe people hate experts or are quick to dismiss anyone who dares to call themselves one for any reason they can think of, to disqualify them and inflate their own egos. But I think that's all counterproductive. If you spent like 20 years doing a thing, you better be at least somewhat expert at it lol.

Often whenever someone quotes how many years/decades of experience they have, I then begin to wonder if they have become complacent and/or adverse to learning, improvements, and change.

That's a lot of negative shit to be assuming based on a cursory comment. If someone was averse to change, they probably couldn't be bothered to stick with anything because just about everything changes. I know I'd rather seek advice from an experienced specialist rather than a rando person with some limited experience.