This article is two years old, and perhaps discord have improved their accessibility, since this user find it more accessible then matrix. Yes, it's a single usercase, but worth mentioning nonetheless.
I think there are other arguments against Discord that haven't been mentioned: data privacy. I know there was an instance where Discord collected user without their consent, and that is enough for me to avoid the platform.
I much rather use matrix or the horridly old IRC protocol than Discord. Or forums. Or just plain old issues!
Second, allowing access to message history is perfectly doable if the invite process involves the inviter providing the decryption keys to the invitee.
You're actually joking with the "inviter providing the decryption keys to the invitee" part right?
The whole point why people use discord is that it's simple, this is a feature that'd only annoy the average person, and every single extra step is a disaster for user retention (look at any eshop study).
Stuff like this is completely irelevant to discord, the tiny subset of people who actually care will and should use Matrix / other solutions, because that's the people they were made for.
Have you tried using Signal on desktop? It doesn't offer history syncing. Cross device for whatsapp for example is also a terrible experience. Unusable for something like Discord.
For a seamless experience Discord would probably have to store the keys themselves, defeating the whole point.
That's because both Signal and WhatsApp don't store the message history anywhere except on your primary device. (plus personal backups) That's why WhatsApp desktop stops working if your phone is off. Because it works by getting your message history, from your phone.
So to get the message history on Signal/WhatsApp in a chat you just joined, someone else already there would have to send you the entire chat history from their primary device. Which might not be on. Or have the battery to spare to stream years of messages to random people coming and going from the chat.
For "a seamless experience" Discord only needs store the message history on their servers, just as they already do, but do so encrypted.
For you to see that history, all that needs to change with how invites work, in that they would come with a decryption key transferred in the same secure way normal messages are. So your client can then access that server-stored chat history and decrypt it.
The difference here isn't that WhatsApp and Signal are encrypted, it's that they fundamentally handle messages differently from discord. Their servers only deliver them. So you can't get the chat history from their servers, because it isn't there.
You'd still have to handle transferring keys across your devices, every time you login on a new one.
Also, searching would probably not work, at least as well as it does right now, since all messages are indexed on the discord side, which they wouldn't be able to do without seeing them. Everything would have to happen on device, meaning the devide would have to store all messages.
IRC has the same problem as discord when it comes to using it for support. It can't be searched. The same questions will get asked over and over again.
With forums and issue trackers, users can find a solution to previously solved issues with a simple web search.