In my opinion, in this kind of big change, it's important for us to make things reversible, and to consider a possibility for us to go back.
I think it's better for us to keep the infra of the mailing list for a while. When we migrate the devel@ to the discourse, we should still keep to manage other mailing lists without migrating to the discourse. It's risky to migrate every mailing list at the same time or in a short period.