I think you misunderstanding **plugins and preferences**.

If you want to override an entire class you need to use preferences:

    <?xml version="1.0"?>
    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">   
        <preference for="Magento\Sales\Block\Adminhtml\Order\View"
                    type="SR\StackExchange\Block\Adminhtml\Order\View" />
    </config>

You can get a good read about the differences between plugins and preferences here: https://magento.stackexchange.com/questions/85277/magento2-what-is-the-basic-difference-between-plugin-and-preference