Do you think it's likely that the behaviour of ~1B people on an existing service changes so much that one component of the service scales another order of magnitude?

That seems highly unlikely unless they launched a new product, or this was a gradual shift over many years.

Yes, new features get added over time. Discord supports servers with over 500k users in them while having a 10th of the MAU of Whatsapp. Whatsapp only supports communities with a max of 1024 users. That's a 2 orders of magnitude difference.

Discord is a completely different product... My point is that it's unlikely users will change their behaviour that drastically. More likely, they'll use Discord for large communities instead of Whatsapp even if Whatsapp added support for communities that large.

It's not completely different. Whatsapp is pretty close to Matrix in what kind of product it is and plenty of people see Discord and Matrix as competitors.

WhatsApp style features are effectively a subset of Discord/Slack/Teams. The difference for collaboration tools like Slack v messengers like WhatsApp includes: large groups, Serverside synced history, Serverside search, large numbers of conversations, group conversations together, group access control (eg RBAC), SSO, threads, gisting, link based conferences, conference broadcasting, integrations with other apps, etc.

Matrix as a protocol provides the building blocks for both messengers and collaboration tools (and more exotic stuff like spatial collaboration apps). Element is an example of a collaboration app (although Element X is being built to feel more like a messenger which happens to also support collaboration use cases - more like Telegram, but encrypted and open and decentralised). FluffyChat is an example of more of a messenger use case on Matrix. Thirdroom.io is an example of virtual worlds on Matrix.

So: I’d say that WA and Discord are very different products. But you could build both on the same underlying primitives (eg a stack of IP, HTTP, TLS, Matrix)

In what way ? users don't just decide to chat more.

Why not? You can connect people with more people to talk to over time. You can give them more things to talk about. You can make it easier to share things with people on whatsapp. You can add streaks to incentivize sending messages even if someone would otherwise not send one.

why ? You ain't getting paid per message.

Because the more people who use your platform the better it is. Having a user that logs on 1 time per month and sends a message to himself isn't a very valuable to the platform compared to someone who comes on the platform every day and sends 20 messages between 5 people every day. Proper who when meeting someone new recommend to talk to them over the app instead of a competitor.