I don't think that will work with any code that came out in recent years. Because it all expects that its dependencies are automagically included via composer.
In practice, every PHP based web application starts with Laravel or Symfony these days. So you are thrown into the composer workflow right away. And it would be a nightmare to fight it.
Web applications come in all shapes and sizes. If you have an application that just mediates data from database and exposes API to frontend then for couple of endpoints you don't even need router or any other packages, PHP has lots of stuff baked in. You just check if the request is GET/POST/PUT/DELETE/PATCH, work with data, output a header 'Content-Type: application/json', send the data and be done with it. Or output a html template with data or whatever. Or you can not use templates and mash it all together in PHP which already is a templating language. You can do it as simple or complicated as you need.
And then if you start needing additional functionality you can start adding packages with composer one by one. Of course it'd be stupid to build your own version of Symfony with it, but the beauty is that you can stop at any point you want if it covers your needs. This sort of thing would be much more hassle with say Java, without using any frameworks.
You can manual include, define your own extra auto loader and use interfaces to quick swap for custom implementations.
Like I said, +15 years. Composer is 9 years old. How do you think we handle this in our own Jurassic times?