There is no point in encrypting the content of these public groups if anyone can just join and get access.
Right, that's what I was saying. There's just some confusion because the original poster was talking about small/medium-sized private chats, and then the replier wrote "I'm not sure I’d want group chats with thousands of members to be end-to-end encrypted" for some reason.
There's no point to encrypting public group chats. There's a ton of point to encrypting private group chats. They should support it for private chats and it should be the default. Or, better, there should be no way to disable it, even.
I think a decent compromise would be just enabling it by default for private group chats, since it'd be costly and pretty pointless for public ones.