It shouldn't just be limited to messaging. An internet where everyone can built a client against Facebook's API, or Youtube or what have you and users get actual choice and control about how they consume those services would be a big leap forward.
Or a wonderful leap backward, in the most positive sense.
In these days of proprietary wall gardens everywhere it might be difficult to remember, but earlier in the Internet that was how things worked. Every protocol was public, documented in RFCs and all implementations were interoperable (barrings bugs/etc, but mostly anyway).
Even a completely obscure protocol can be reverse-engineered given enough time - in fact if you search on GitHub you can already find a lot of client libraries for proprietary services.
The problem is that at the moment the platform owners intentionally detect usage of these alternative clients and ban their users or abuse laws such as copyright to block their development & usage.
Peer-to-peer communication in WhatsApp in the network topology sense happens where possible when making Voice and Video calls, as this is probably WebRTC-derived (it is WebRTC in everything else these days), which concretely involves some kind of call signalling, then p2p setup to talk RTP if possible. This is not Signal Protocol or Noise: it is most likely the S in SRTP with key agreement done over the Signal Protocol. In other words, no key ratcheting between voice or video packets. I'm actually not sure if the session key is ever changed for a given call. To make this clear: call setup happens via a central server but the media streams will go from your IP to theirs directly, if possible (or proxied via WhatsApp if not). The reason for doing calls p2p like this is where possible is to reduce latency.
This is also, last time I looked, true of Signal. We are good at end-to-end text. We are less good at voice/video, particularly voice/video group calls that might not be p2p-able and rather require the server to do something with the RTP streams.
Now, what you're actually missing is that WhatsApp was in its early days based on a fork of ejabberd, the Erlang XMPP Server, with if I understand correctly custom extensions. Thus WhatsApp actually was at some stage somewhat compatible with open standards.
We've also kinda been here before. Google Talk used to interoperate with XMPP just fine and at one stage my own XMPP server could talk to my friends on Google Talk and they'd pretty much not notice.
I agree however that it would be better to have a new protocol that starts based on end to end key agreement like Signal/Noise, rather than use XMPP. Or perhaps use XMPP _inside_ this protocol. This is because "opt-in" crypto is a disaster that probably has happened. Signal and Noise are also missing what the body of those messages should look like and standards for agreeing for example calls, media transfer and so on, basically all the non-crypto parts.
Seems WhatsApp does use noise pipes for some some long running connections. Just checked their doc to be sure.
So, I wouldn’t assume it’s great for end users without digging into the details. Don’t forget the last time they did privacy regulations they created an unending wave of click yes to accept cookies.
PS: Looking at rapid downvotes I see people disagree, but mandatory interoperability would presumably force them to accept SpamNetwork101, SpamNetwork102 … etc.
And even if you for some reason don't want to restrict your requests, you'll probably still be fine - Gmail handles protects me from spam pretty well.
Is say MMO chat a social network?
New person goes to chat requests, they communicate through another channel they've sent you a message, you go open the graveyard of chat requests and accept theirs.
Using this model it would better to have multiple lists though, or at least tagging within the general contacts list, so tag-based lists could be allowed by certain apps (to keep business and personal messaging separate for example).
WhatsApp replaced SMS as a free alternative with media. Sms is just a protocol. It is not necessary that a replacement is walled garden, especially not under the sole guise of spam protection - something that is being done very poorly anyway.
Depending in the specifics everything from Yahoo! Messenger to MMO chat either needs to get shut down or made interoperable.