With my Wikimedia admin hat on, I'd not install Ponder in Wikimedia Phabricator because https://xkcd.com/927/. See https://phabricator.wikimedia.org/T31923 and https://phabricator.wikimedia.org/T155678. Social fragmentation problems do not get solved by more tools, but by consolidation via less tools.
Part of the argumentation is based on a wrong assumption as Wikimedia Phabricator is not "used in numbers". Please compare the number of Wikimedia wiki editors and Wikimedia Phabricator users (minus some developers only). There are still many on-wiki folks who are uncomfortable using Wikimedia Phabricator.
I agree with nearly everything else you wrote though, e.g. that Wikimedia's landscape is very fragmented. I think this would need a huge huge discussion across Wikimedia communities which nobody has the capacity to plan and lead (my personal interpretation) in order to get to a rough concensus. And it would require switching off systems and Wikimedia (whether Foundation or movement) has been bad in doing that.
Even if using Ponder was rather limited to developers, do you propose switching off Wikimedia's `wikitech-l@` and other technical mailing lists, for example? How not to increase fragmentation is my biggest concern.
The answer to "What's the alternative?" comes at the end of a long process, after defining a problem, discussing with stakeholders their must-haves and should-haves, and analyzing and testing software (plural) whether it covers the requirements...
(**Ponder does not allow me to add a comment as I have already answered this question.** So I keep editing my single answer, which feels wrong...)