At this time, no-one is asking for such a change (most likely because they’re simply unaware that something like Discourse exists), and the “powers that be” in the Apache Spark community are open to the idea but doubtful that it can be made to work within the ASF’s requirements.
My thought was to try selling the community on Discourse by offering a low-friction, low-commitment way to interact with it that still played nicely with the existing ASF-managed mailing list. Some kind of bridge between Discourse and a totally separate mailing list might just be the solution, but it’s probably too much to ask of Discourse at this point if such a thing doesn’t already exist.
It sounds like your recommendation is to not try this soft approach of offering a totally optional alternative and instead get buy-in directly from the leadership and community before trying anything.