I understand the rationale for this under normal circumstances. I feel that there are mitigating factors in this instance:
1) The current email client has substantial issues with regard to design, robustness, functionality, and quality.
2) The K-9 team has really done a lot of work to make the client meet a minimum standard of functionality and quality.
3) The K-9 team has done work to maintain compatibility with the core codebase (preserving the com.android.email namespace), so actual patching should not be an issue, but K-9 is significantly ahead of the default email client, and the code reflects that. While it should be possible to submit small patches, it is a huge amount of work to "replay" all the changes, and some of those changes conflict with future changes, etc.
So, understanding that small patches are preferred, is it possible to accept a (set of) larger patch(es) to bring things back together? I don't believe that it is a good long term strategy (from an android platform perspective and from the K-9 perspective) to have a fork, but some amount of "meet in the middle" would grease the skids, so to speak.
Thoughts? I'd be glad to arrange a more in-depth discussion between the email client team and the K-9 team to facilitate de-forking and ongoing coordination.