Helpers are not final classes, so it is assumed we can extend them.
If, for any chance, you needed to extend ProgressBar, it would fail.

This test

use Symfony\Component\Console\Helper\ProgressBar;
use Symfony\Component\Console\Tests\Fixtures\DummyOutput;

class ProgressBarExtensibilityTest extends PHPUnit_Framework_TestCase
{
    public function testStart()
    {
        $progressBar = new ExtendedProgressBar(new DummyOutput());
        $progressBar->start(10);
    }
}

class ExtendedProgressBar extends ProgressBar
{

}

fails with the message

PHP Fatal error: Call to private method Symfony\Component\Console\Helper\ProgressBar::getStepWidth() from context 'ProgressBar'
in vendor/symfony/symfony/src/Symfony/Component/Console/Helper/ProgressBar.php on line 573

It happens because the private method getStepWidth is called in a closure:

'current' => function (ProgressBar $bar) {
      return str_pad($bar->getProgress(), $bar->getStepWidth(), ' ', STR_PAD_LEFT);
},

It works only inside ProgressBar but ExtendedProgressBar has no access to private methods of the parent class.

My proposal is to change a visibility of ProgressBar::getStepWidth to protected.
All other ProgressBar methods that are used in the same context (in initPlaceholderFormatters) are public.

(I'd work on PR if there are no objections or other ideas.)