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.

When you download the imagery, the data is in UTM rather than Web Mercator.