My two cents – The shortcodes / page builder add-ons are great ideas but shouldn’t require GeneratePress and should work without it to avoid “theme lock-in”. Even if they are implemented as a plugin, having GeneratePress dependencies will produce the same or similar result.
If you want an example, all you have to do it look at Divi by ElegantThemes to know why you’d never want to create something like this. It looks good, works well but once you begin using it, you’re stuck with it. Your content is now hostage your chosen theme even if it’s a beautiful abomination.