Thank you @nicolas-grekas.

@nicolas-grekas Could you please explain what is the sense of making this methods final?

@ossinkine see PR description. Why would you need to extend them?

@nicolas-grekas PR description just says you propose to make them final but does not say why.
For example, I want to override render method in abstract controller to transparently set default parameters or set custom header to response:

abstract class MyController extends Controller
{
    public function render($view, array $parameters = [], Response $response = null)
    {
        $parameters = array_merge(['foo' => 'bar'], $parameters);

        return parent::render($view, $parameters, $response);
    }
}

So children controllers continue to use render method.

By the same logic, other methods can be overridden

@ossinkine after #24722, the method will have a new signature (type-hints, as stated in the above description), so that if your code extends the methods, it will break. That's why.

@nicolas-grekas I understand the new signature breaks BC and I've have to fix my code. But why I can't override the methods in future when my method signature is fixed?
I will need to create one more method like renderWithDefaultParameters, right?

I'm surprised after try 3.4 beta 2 and discover this like ossinkine.
IMHO add type-hints in the future couldn't be a reason to mark this method as final. In the next version we can fix our signature to adapt to parent type-hints.