I don't see why that's a problem. They always can start their own conversation about the same topic if I, the conversation starter, don't find their contribution to my conversation valuable.
Unless, of course, there's some kind of mechanism that gives threads with higher engagement more prominence? That might create an incentive to have more inclusive conversations. Although that might incentivise trolling too...
EDIT: Perhaps combine the above with a reputation system, where you can see how ban-happy an original poster is. Since people don't like to waste their effort writing a reply that just gets ignored, ban-happy posters would be penalised by lack of engagement. Then platform provided moderation could just become a kind of 'meta-moderation' - basically just banning people who try to game the system (e.g. posting threads saying 'please reply so I can get my ban percentage down').