There's not much in PHP 5/7/8 which precludes just doing some raw db queries and including some templates.
The biggest 'drawback' might be that there's not one name-branch 'framework' which takes this approach (indeed, it's the opposite of 'framework' thinking). But if there was some project that documented/demonstrated 'best practices' for 'non-framework' projects, it might help reduce the "I need a framework because raw PHP is so messy" objections.
1 - security-safe and type-safe from the beginning
2 - none of the additional programming paradigms. PHP wants to be everything in one language. I know you can still use PHP "basics" like that, but it's not going to be easy to find tutorials etc. showing that
The main point I wanted to make was not about the index.php file. In other words, I'd like a PHP3, but a clean, safe, very well-designed one.