Nothing is wrong with plain old HTML. Going headless gives you frontend freedom. It means integrating your content on any platform you like: web, mobile, smartwatch, fridges and toasters (if they are smart enough). It's interesting that we mostly see people using a headless CMS just for web.
Why?
Because they can pick the frontend stack they like! React, react-static, Gatsby, vue, nextJS etc.
You can pick the stack you like most. It is much easier to find a react developer than someone who knows how to write good drupal templates.
Why GraphQL? Because it's awesome! I can highly encourage you to just try out a GraphQL API and see yourself. It is designed for fetching related data efficiently and it will give you a huge boost in development productivity, which means lower development costs and faster time to market.
Why?
Because they can pick the frontend stack they like! React, react-static, Gatsby, vue, nextJS etc.
You can pick the stack you like most. It is much easier to find a react developer than someone who knows how to write good drupal templates.
Why GraphQL? Because it's awesome! I can highly encourage you to just try out a GraphQL API and see yourself. It is designed for fetching related data efficiently and it will give you a huge boost in development productivity, which means lower development costs and faster time to market.