Calculated misery by Tim Wu explains a lot of this. Goes on to show how airlines and other industries as well. I think the paper was published in 2014.

Ctrl-y to paste what Ctrl-u deleted or cut

I might have missed if it was in the article. Sorry in advance for that.

Do we have any idea regarding the top 3 or top 5 reasons that drove potential contributors away? Not in terms of hypotheses, asking about actual studies, interviews, surveys, etc

From what I can see on the F-Droid description, Focus connects to iTunes, presumably in addition to the same places Antenna does, for podcast search indexing

Been a few years since using Emacs extensively. From memory, IRC is a good fit for what you are after for texts and some emojis. No clue regarding multimedia messages.

If IRC is acceptable

Make your own channel on say, Libera chat, set your own rules for how long those messages are retained. Make a user for each of your devices. You are set. I’ve used ERC a few years under Emacs. Also used GNUS for reading and writing emails from the big providers.