Lightest setup to sell only digital content with no physical products?
You could get by using Drupal Commerce with just the core modules, some custom Rules, and your preferred payment gateway module. The modules and features you pointed to (shipping, inventory, etc.) and the ones you didn't point out have nothing to do with Drupal Commerce itself. They're all optional contributed modules and don't have to be added if you don't need them.

It's possible you saw a demo site running Commerce Kickstart 2.x. That is a distribution of Drupal that includes Drupal Commerce and a wide variety of contributed modules to make it function more like a robust eCommerce application out of the box. At its core, Drupal Commerce is much lighter weight, and I think you'll find its compatibility with Rules combined with the Content Access / Access Control List modules to be very helpful in selling access to digital content on your site.