The biggest thing missing from this analysis is "is there a business case for supporting those 2% of users?". (Maybe, maybe not.)

The second biggest thing is progressive enhancement. The author picked a CSS feature (nesting) that is basically all-or-nothing: the site will basically be entirely broken for those 2% if you swap Sass for native nesting. Most features aren't like that; maybe the site won't look pixel-perfect on old browsers, or one bit of functionality won't work, but by and large it will still be functional. In those cases, I think it's a much easier decision in terms of where to draw the cutoff.