Top posting is fine for really shallow conversations.
As soon as you attempt to have more complex conversations, say about subtle technical issues, it falls flat on its face and inline quoting becomes far preferable.
In a way, this is a culture issue similar to the whole maker schedule vs. manager schedule discussion. A lot of management types engage primarily in shallow conversations[0], and so they may think relying on Outlook is fine, while being totally unaware of the damage it does to the engineering part of the organization (and possibly to themselves).
[0] In part this is inherent to a manager's role. For example, in many cases a manager's email may simply be the message "yes, dear direct report, you have formal permission to proceed with your plan". There's just inherently less deep thinking in a management role, when compared to engineering roles. Of course, deep thinking should occur in management as well in plenty of places, and the best managers do recognize that. But unless they come from an engineering culture, they're probably not even aware what they're missing with top posting.