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: Magento2: what is the basic difference between plugin and preference?
