If some people want to talk about a (sub)topic and others don't, why not
create a separate mailing list for that topic?
Then the discussion can continue unimpeded, without destroying this mailing
list.
Because things like this DO destroy mailing lists, because people unsubscribe
from a mailing list which is dominated by a topic they find unworthy. Let's
give such people a different mailing list to unsubscribe from instead, and
divide topics between the two mailing lists appropriately.
regards,
ajr