Most users are already using Signal, WhatsApp, Telegram etc. I don't think it makes sense to sacrifice privacy to make Riot work similarly to them when the whole reason to use Riot is so that you don't have to make such sacrifices.
How does Discord works? It's pretty popular and I don't think they do that. Somehow it doesn't seem to be a problem for them.
Discord is just one specific use case of an app, while Matrix is trying to support all possible ones (e.g. WhatsApp style behaviour).
However there's a broader point that it doesn't make sense to imitate the competition when the thing you're copying is precisely the reason why one would want to leave the competition to your solution.
Also, the Identity servers are part of a closed cluster where data is replicated. We are aware of vector.im and matrix.org but you did not answer the following question of the research document: is there other servers in that closed cluster? If yes, which?
IS lookups are shortly going to require auth, as per the OP (even if just to check whether the API user has agreed to the server’s T&Cs)
There are no other servers in the cluster.