Facebook have always wanted to replace the internet entirely with a system they control. Hence the walled garden approach with incessant prompts to signup to view content, the use of address book import to spread virally, the use of beacon to track and announce users activities, the use of like button js to track activity, the launch of Facebook apps and single sign-on to attempt to corral web developers within their ecosystem and make them dependent, the launch of a competitor to email/IM etc etc.
This latest move with email is simply the latest in a long line of moves consistently attempting to steer users into spending all of their online time within the confines of Facebook, it is entirely consistent with their past behaviour and we should expect similar moves in future.
So the arguments about how they could have done this differently, or whether Facebook has made some mistakes here are largely irrelevant - they will always push to own your data, email, photos, and digital life, because it's in their DNA, this is not a mistake, it's a pattern of behaviour - they can and will control your digital life if you choose to hand it over to them. Consider the quote from FB on the way their FB email works:
Regarding the "email loss" this may actually just be confusion around the Messages Inbox: By default, messages from friends or friends of friends go into your Inbox. Everything else goes to your Other folder.
All messages which do not come from FB contacts go into a folder which is effectively a junk folder - they want you to use their email for everything (hence the recent changes), they want your address book to contain nothing but FB contacts, they want you to forget email even existed, and they want you to encourage your friends to join FB so that you actually receive their messages. That won't work for all their users of course, but their hope is that they keep enough of the user base on board that they can dominate the internet and replace email with FB and the web with a network of sites where they control login and user info and all roads lead to FB.
So replacing email with FB mail wherever they can, hiding other contacts and trashing user address books is not a mistake, it's part of a broader strategy, one which is not in the best interests of their users, but which they hope most will go along with anyway.