Alternative approach to consider: don't rebuild older posts. Transform them from Markdown into HTML in situ and then never worry about recompiling that post again. You could do this by either keeping both the Markdown source and the output document around, or by crafting the output in such a way that the compilation process is reversible—so you could delete the Markdown source after it's compiled and derive it from the output in the event that you ever wanted to recover it.

traditional compilation foils "symmetric authoring"

Readable how? I think the better approach over what we do now, where we run source code through what are essentially compilers for making websites and then treat the output similar to object files—i.e. opaque blobs that are unsuitable for anything other than either (a) executing/experiencing, or (b) publishing to others—would be to pursue non-destructive compilation. So after the Markdown is compiled (if there is any compilation step at all), you don't have to keep the original sources around. The tooling should be sufficiently advanced to work with the publishable form as input, too, and not demand only Markdown. In instances where the Markdown files are kept around because the spartan experience of opening a plain text file where the content is almost entirely unadorned by formatting concerns is the preferred way to get things done, the tooling should be able to derive the original Markdown (or a good enough rendition) from the output itself. HTML is rich enough to be able to encode the necessary structure for this on the first Markdown-to-HTML pass. Systems that implement this kind of thing could be said to support a sort of "reversible Markdown", making the publishable form a suitable source of truth—a treatment that is right now reserved only for the originals that are collected and kept off-site.

Make the writing and editing experience more like Word or Google Docs and less like LaTeX.

(NB: the compilation "object" format here would, much like triple scripts, be another form of human readable source.)

As it should be. (This should be the default. The NodeJS/NPM's Webpack addiction needs to be curbed—or confronted with a reality check.)