Maildir storage format is resistant to bit-rot because it stores each message in a separate file, and uses filesystem directories for mail folders. It's widely supported by user agents (mail readers) and IMAP/POP3/SMTP servers, so you'll never be stranded by the actions of a single software vendor. Finally, it's easily searched using everyday unix tools - find, grep, sed, awk, etc., and you can use the full-text search engine of your choice for speedy searches.
That's crazy talk. There are many threats to the health and vibrancy of an open source project, and being backed by a commercial company is not a reliable indicator of danger.
Consider the behavior of the project maintainers and planners. Do they engage the community on key issues? Do they accept outside contributions? How are conflicts resolved?
The issue isn't who funds the developers, but the attitudes and behaviors of project leadership.
You maintain your copyright for Asterisk contributions if you used Digium's contributors' agreement.
Read carefully: https://issues.asterisk.org/view_license_agreement.php
Most contributors grant a "perpetual, worldwide, royalty-free, irrevocable, non-exclusive, and transferable license" to your contribution that allows dual licensing. Unless you specifically disclaimed your copyright, you still have it.