You present an interesting point and certainly we do encounter (fairly regularly) this exact situation.
I don’t consider the Layout Designer as a competing tool to hand crafted HTML/CSS as this would be a losing battle - you’ve only got to look at IDEs for webdesign to realise that. The Layout Designer is a convenient tool for a non-technical person to make a passable design without involving a professional designer if they choose not to. This is an important part of what the project provides - the idea that these things should be accessible in this way.
Another important part of what the project should provide is the platform for designers to create content that is better than what they can do in the GUI - be this with Video or HTML/CSS. Our route to doing this is with the Module system, which I absolutely accept could do with improvement to make it easier to use, either through changes to the way it works or through better documentation.
The module system has a bunch of structure around what you can and can’t do, so that Xibo can ensure things are delivered to the Players reliably and in a sustainable fashion, but otherwise does (I think) meet your goal of using hand crafted HTML/CSS, other resources (JS, Images, etc) and provides a framework for allowing your users to configure the data using the Widget Add/Edit form.
How we can improve this system further and make it more accessible is a question i’d love to have answered - as you say we’d be on the way do becoming the WordPress of Digital Signage 