So, why should one use the next generation? Because it is not tied to R or Python? We're all using one or the other so that doesn't sound like a convincing reason to completely change my workflow.

I think if you are happy with the workflow/output you have there is no reason at all to change. w/r/t to websites and books we have put a lot of investment into features for both (https://quarto.org/docs/websites/, https://quarto.org/docs/books/). If you compare books with Bookdown vs. Quarto vs. JupyterBook I think you'll find that each does some things better than the others (so again if you are happy w/ your existing tools no reason to change).

Thanks for the response. Is there any plan for Quarto to support live code, e.g. the reader can change the code and update the output? Or a feature that makes it easy to have a comment section for each page? I think these things are either not supported or not trivial to do with Bookdown or JupyterBook.