An important criterion for community migration paths is the level of migration that occurs already by moderator action.
There's no "right" level for establishing a migration path, but tens or hundreds of migrations with a single digit rejection rate seems safe. Single digit migrations in the last 90 days and/or rejection rates that can order a drink without ID are signs that the path will be a waste of time to establish.
Opening those migration paths is fraught, as sometimes low-quality questions that should be closed get migrated instead (a constant frustration already on the part of ELL.SE members), or questions that are answerable here get migrated unnecessarily. Moderators are expected to be more knowledgeable and careful than the community, on the whole, so if they do not deem many questions worth moving, there is no cause to throw that open to a wider constituency.
A SEDE query for questions migrated away turns up these results since the EL&U epoch:
Site Count Latest Migration
ELL 3676 2019-08-23 13:00:32
Writers 166 2019-08-22 14:55:45
Linguistics 65 2019-07-01 03:07:07
SuperUser 21 2019-01-14 13:21:33
Philosophy 19 2019-01-23 17:11:37
(Writers.SE was renamed Writing.SE in January 2018, after several months of discussion; presumably some SEDE table needs updating. Literature is at #6, with 17, but I think that includes migrations from the failed 2011–12 incarnation as opposed to the currently constituted Lit.SE).
Since this is skewed somewhat by the age of the available targets (why else would we have more migrations to Mathematics.SE than to LanguageLearning.SE?), 10K users can also see the 90-day migration statistics:
Site Count Stats
ELL 175 6% rejected
Meta.ELU 12 16% rejected
followed by eight singletons.
The migration rate to Writing is two orders of magnitude below that of ELL by either measure, which doesn't make a compelling case in my view, though of course I am just an ordinary community member with no larger insights.
If you come across questions that would be better-answered at Writing.SE, it sounds like we should step up our efforts to flag them and have moderators move them. The case for community-driven migration will be much stronger once we can demonstrate that adding that path will be beneficial for both communities.