I can’t see how. The main advantage of the email delivery is to reach the people who don’t actually visit the community frequently. With a delivery to the message inbox, the number of people reading the newsletter would go down drastically, because many would never log in to read the message—or they would get an email notification for the message, which defeats the entire purpose of moving the newsletter from email to the inbox. There are other ramifications as well. Each message is stored separately, so a newsletter to 20,000 people would be stored 20,000 times. (Preventing that would probably require major changes to the PM system.) And people would be temped to reply, as it is a personal message to them, but as admin I might not be willing to read hundreds of private replies …
If I want to reach the people who are already online, I could just post news as a forum post, an announcement, a Pages article … there is no need to make it a personal message.
If too many people unsubscribe from the email newsletter, that just points to the newsletter not being interesting enough.