Nice, if XMPP works for you, but I hope do not use Pidgin or Adium nowadays. They still work, but AFAIK, they do not support newer XMPP features, such as encryption using OMEMO, HTTP upload, message archive management, etc.
There are nice, modern XMPP clients like Conversations on Android, Gajim on Linux and Windows, etc. that do support the latest XMPP features and are much more pleasant to use today.
I have to agree and disagree. I agree, that XMPP clients are not very good. And horrible on iOS (and MacOSX). I disagree, that Matrix has good clients. Riot is an OK web client, but too slow and too confusing for my taste. And native desktop clients, like Nheko or Quaternion, are not yet very useful. XMPP has many more mature clients at the moment.
On Matrix practically everyone is matrix.org. The other public servers I tried are relatively slow and suffer hickups when joining rooms.
On XMPP, many people run servers in their companies, projects, or homes. Which are the "top 3-4 XMPP servers"? I probably do not even know them.
But you are right: XMPP is used mainly for IM, i.e. one-to-one dialogues or smaller, non-public chatrooms. I'm not aware of any huge, public chatrooms in XMPP, and for that I'm on IRC — via a gateway from XMPP called biboumi.
Where does "order of magnitude" come from? In my experience, XML parsing is about three times slower than JSON and around two or three times the size. For most complex applications, this is not a show-stopper, because the bottleneck is elsewhere.
XML was a hype in 1999, when XMPP has been invented and JSON was a hype fifteen years later. When people invent protocols they just use what is trendy in the moment.
OK, you want XMPP. But without the many XEPs, something with a monolithic specification. And you like to use the latest trendy hipster stuff, like JSON over HTTP instead of XML. Then you get:
Monolithic, Awefully Trendy Re-Implementation of XMPP (M.A.T.R.I.X.)
Which is also a dedication to XMPP, which has been invented in 1999, the year of the Matrix film.
I'll stay with XMPP until they take it from my cold, dead hands.