Absolutely–A hard migration to Discourse is almost definitely socially impossible. That’s why I am interested in using Discourse as a new, third-party interface to the existing, ASF-managed mailing lists.
That is, people will still be able to use the mailing lists as-is, but if they so choose they can instead use Discourse and gain all the added functionality that Discourse offers. In the background, Discourse and the mailing lists will relay messages to each other via some bridge.
I think such an approach, given technical feasibility, is socially possible as it does not require the ASF to change the role of the mailing lists. The ASF will just need to support a two-way bridge between the mailing lists and Discourse.
In other words, the ASF is just providing a hook into its mailing lists for third-party services like Discourse to use. Apart from supporting that hook, the ASF doesn’t need to know about those external services. Similarly, mailing list users can continue to use the mailing list as-is if that’s what they prefer.
And the rest of us can use Discourse (perhaps with some restrictions on functionality) as a better interface to the same mailing lists.