[acdha]

I think this is a little less bad in context: in 2005 Gmail was a year old. Most people used a dedicated email client app such as Outlook or Mail.app so in your flow it would be far more defensible and his view was focused on corporate users. That makes the first point a little more reasonable:

1. Your desktop application shows a list of attachments in the navigation chrome where a message can't display content.

2. When you click on something in that list, Internet Explorer or Firefox seamlessly logs you into the server using Active Directory.

Storing things on a server was also more relevant in the era where space was limited and services like Exchange were famously difficult to scale or customize. If you didn't have good tools to retroactively yank a message out of everyone's inbox when your AV signatures were updated an hour after it arrived, storing it on a server you controlled had a certain practicality.

Your second and third points are spot-on, however, and really hit at a key principle too few security teams appreciate: normalization of deviance. This approach fails badly in the real world where IT security says “don't open attachments from people you don't know” and everyone's manager says “oh, it's totally normal to get passworded ZIP files from the HR services subcontractor. Open it, we have a deadline!”. The real lesson here should be defense in depth so your organization's security isn't jeopardized when one person opens the wrong email.