The ecosystem is just R packages, right? There's also RStudio integration with e.g. rmarkdown, knitr, quarto etc. but presumably all of that can also come in positron.
Yeah, I'm a huge fan of rmarkdown & knitr, not sure if that's Positron compatible—I know that I get RStudio menus/dropdowns that come with those integrations.
(FWIW, I'm the technical lead on the Quarto project)
RMarkdown isn't going anywhere! Quarto exists to bring the RMarkdown experience that folks love to a broader set of users and contexts. It is true that we try to keep the .qmd experience in Quarto pretty close to the .rmd experience in RMarkdown, and it is true that Quarto does things that RMarkdown never will. But it's not the case that "RMarkdown is being phased out and replaced with Quarto".