Indeed, I pulled 500 out of my hat because that's roughly stock market history. But that kind of reinforces my point, if we only have 100 years that's very little, especially considering how fast things change. Early 20th centuries very few industries were publicly traded, even amongst those that existed at the time. And even beyond the industries, other things have changed a lot like regulation, taxes, accounting rules, management style... surely the market takes all of those into account, one way or another.

Yeah, the thing that makes trading and investing different from most other disciplines is that the distributions are completely non-stationary and are changing all the time. There are some "stylized facts" (that's the term to search for) so use those to at least ground your model but you won't make any money from that.