Jon and I discussed the arrow and folder icon when redeveloping the default theme. We opted for the arrow and folder for consistency with the underlying platform. You'll notice macOS and Windows both use the same pattern. Additionally, it leads to the folder and filename labels being aligned. Without the folder icon, the left edge of labels is not aligned.
But yes, this is leading an area where different people will have different opinions. Obviously not everyone is going to like the Default, or Adaptive themes, but so far the feedback on the Adaptive theme has been overwhelmingly positive. We are now shipping a far superior out-of-the-box experience, especially on Windows and Linux HiDPI setups.
Overall I'm pretty happy that we are shipping a unique, consistently designed theme that still has the feel of Sublime Text, while embracing recent UI design trends. All of the icons and assets were designed to be pixel-perfect at 1x and 2x, we use system standard fonts for labels, and we provide an extensive theme engine so people can make it look just the way they want. Just the fact that the Material Theme, Boxy, Soda and Monokai Pro exist means that people can have a powerful, efficient text editor that looks slick too.