Windows Defender Application Guard Standalone mode

I think perhaps you misunderstand the intent of the feature.  I see primarily it as a sandboxed browser session that effectively runs each page in a VM, therefore eliminating any possibility of attacks affecting the core OS.  The features about favorites, history etc. they talk about and say they are being implemented in a later release.  As for enabling by default, I am sure this will be a Group Policy preference that organisations can set as they need.  Some business has VERY critical data that cannot be compromised in any way, so this is a worthwhile feature and it has been, in my experience, the very high-end employees that are most likely to be fooled by website attacks, spoofing etc. so intelligence, age and wisdom are irrelevant with modern IT attacks :-)  I do agree it will be a useful addon to the novice home user, or Grandma, but let's help MS get the feature tested and stable, then perhaps the rest will come.