People used lavabit because it offered convenience, not because it was the most secure option.
Using pure open PGP is inconvenient. It means you need to manage keys and backups manually. It’s also not integrated with a mail client, so you have to download the blob, decrpyt it and read it locally. I don’t think there are any open source implementations for mobile devices. And as heartbleed demonstrated, even open source is no guarantee against vulnerabilities.
