I don't feel like that is a particuarly good example compare with.
Forums are a gathering place to interact with and meet new people, forming friendships and relations. This is true regardless of if its a small hundred member community or tens of thousands. A degree of private chat is to be expected, but generally people stick to - or at least used to, stick to threads. Maybe they jump around lots of different ones chiming in on random topics related or unrelated to the theme of the forum, or perhaps they stick to one or two and spend a lot of time there.
A degree of private chat is to be expected. I undersand that, I talk to my friends in private as well and prefer others not to interrupt but I do so on Discord, which is an instant messaging service and prior to this I used Steam. Now I realise not everyone can access these but its important that I bring attention to the difference between them and forums.
We'll take a discord server for example. Lets use the Terraria one. People join there and chat and its pretty chaotic honestly. But there's no private channels, so if you find people you get along with you add them to a private, group chat or your own server and talk there when not posting on the Terraria one. This is effectively taking place outside of the Terraria discord.
Now, we'll talk about the same thing but on the forum. If you want to talk away from others or privately your options are Profiles or Conversations; it should be noted that both of these options are still on the forum. But once again, a certain amount of private chat is to be expected. This is a forum and forums have never been intended to be a "private chat room" Maybe some forums have private sections that only allow certain usergroups in, but its not quite the same thing as a conversation created by a group of users.
Certain people have been using conversations basically as a private chat which isn't really wrong, but when used to quite the excessive extent as it has been... it begins to raise concerns especially when people have vastly more messages in these conversations than anywhere else on the forum - Are they using the forum for its intended purpose or is it just a convenient tool to use as a private chatroom which forums are not. A line has to be drawn somewhere at this point and we picked 15,000 as that that line, deeming it appropriate when comparing to other active users that were not abusing the conversation feature.
The forum is a public place, not a private group chat. If you want to talk to people do so normally, not behind private locked doors - that goes against the community aspect. We don't want to stop people from talking privately but we want to discourage it being used quite to the extent mentioned. We've got users with notably higher post count in conversations than their pubic count as seen in a few of the previous pages. While I'm sure some of that is private but it can't all be, and if it is then I'm afraid a forum isn't the appropriate place to use for it.
If you are actually here to use the forum then talking it public shouldn't be such a revolting idea that you'll threaten to leave the forum if you can't talk in private. I understand the upset and I'm hoping people calm down and look at it more objectively. We haven't banned anyone or taken away any forum sections, off-topic and roleplay is still there, as are the general sections. Granted off-topic still has the per-approval basis when making threads but we've had that for a while to reduce spam and we're pretty quick about approving or disapproving threads there and informing the user of our decision, its just so we dont have 15 different threads about the same thing.
There've been a few strange points brought up like conversations being a way to subvert the rules and get away from moderation, this isn't the case and if you're using them for such things you're really putting your account at risk, i'd advise that you don't.
Something else brought up is that maybe they are threadbanned from where they usually chat. And I'd apologise for that but, kind of their fault. I consider myself and my peers to be quite lenient and we quite often give people many clear warnings on what behaviour isn't allowed on the forum. I myself follow up every report with a DM explaining things in clear detail and offer to answer any and all questions, despite that we still have people who refuse to cooperate and ultimately end up being removed from a thread due to their continued behaviour. We do offer appeals in this regard, we have a section for it but we also don't mind you being more formal about it; if you'd like to have restrictions or points removed you should absolutely contact a member of staff or report yourself with a message and we'll contact you.
But I'm getting off-topic here.
To put it in TL;DR...
The forums are a community space, not a private chat room. It is unintended for users to have the majority of their activity in private. We want to encourage users to use the forum in its intended manner rather than using it as a convenient tool. For those that have a great deal of private messages but have not been abusing the system in the same way, we apologise for the inconvenience but we decided it was an important decision to make for the betterment of the forum as a whole.