The first paragraph frames one question, and the second paragraph frames an entirely different question. I think you are implying that they are the same, but they're not.

As for your first question, it's a matter of opinion, based on what you want and need. My guess is that somewhere, the point of diminishing returns is between $500 and $1000, but just for me. It's elsewhere for other people.

It would be hard to find an equivalent bike to yours, because yours is well made but modern components function better because of the features and designs.