In many browsers (e.g. Safari), the scrollbar is hidden when not scrolling, while the reading progress bars I know remain visible permanently. Furthermore, the scrollbar describes the whole vertical length of the website, not necessarily that of the specific content you are reading. I do personally observe a trend of the ratio content length vs. full page length increasing (e.g. increasing use of white space in web design).
This is not commenting in favor or against reading progress bars. But if one concludes that reading progress bars are a good idea, I welcome the widespread adoption. The spread would mean that more users become familiar with the design pattern and its meaning.