> The issue with that is that when there is no "leader", there is also no way to guarantee kicking someone out.
Why is that an issue? It's a fundamental fact about the world that your software will never address. No matter what options you purport to provide, you can't stop people from telling other people what messages they received.
In a decentralized system, messages are sent to a list of recipients. If you don't want someone to receive your message, you can take them off the list of recipients that you send to. But if you send a message to party B, and they recommunicate it to party C, there's nothing you can do about that. The only solutions are (1) to stop communicating with people you don't trust; or (2) to have the guy you want to kick out of the chat group kicked out of the world.
But notice your own option: "stop communicating with people you don't trust." For a group, kick is an option: everyone stops including X, at once.
And that's where the leader matters -> to make that option executable as a group. Without an agreed authority, it's N separate choices that have to stay consistent forever: one member with a stale roster keeps X in the loop by accident, or X kicks Y while Y kicks X and there are two rosters claiming to be the group. One signed kick says "we stopped talking to X" for everyone.
But then again, you're right. If 4 members want to kick out member#6, but member#5 doesn't want to, there is nothing we can do to stop member#5 from sending everything to member#6. That's not a software-solvable problem.