Equirectangular is not a projection. For all of web mercator's faults, it at least preserves direction, which is nice when you're looking at streets/etc locally. Regardless, all of the web mapping stuff standardized on web mercator ages ago. Like it or not, for web mapping, you're stuck with web mercator, unless you want to serve your own streets/etc.
It's the same projection most online maps use, which is a bummer. Equirectangular (https://en.wikipedia.org/wiki/Equirectangular_projection) is much easier to grok (IMHO) and is supported out of the box by D3.