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)