> I'm defining closed networks as ones like Facebook Messenger and iMessage and such.

It's odd you put Facebook Messenger in the protocol group that killed off Trillian as it has always supported that. When Facebook Messenger launched it was XMPP compatible and nowadays when it has moved to be fully proprietary it has remained supported through reverse engineering (just like most protocols Trillian supports).

That example aside your further examples of "open protocols" Trillian supported are even less open and more hostile to 3rd party clients than Facebook Messenger:

> The "closed" networks that Trillian accessed still used open protocols. But my progression was AIM, then ICQ at the same time, then Trillian which could do both plus the few people on Yahoo messenger and GTalk and IRC

AIM used their proprietary OSCAR protocol, which ironically the "O" stood for "Open" but they never actually distributed a spec/standard and actually went through great effort to prevent other reverse engineered 3rd party clients from being compatible over time. OSCAR had to be reverse engineered no different than (newer) Facebook Messenger or Discord or so on today.

ICQ was where OSCAR was developed (under ownership of AOL). All of the above applies and it intentionally broke unauthorized 3rd party clients many times.

Yahoo Messenger used their proprietary YMSG, had to be reverse engineered, and was often hostilely changed to shake 3rd party clients.

GTalk was better but that's because it was intentionally XMPP compatible. Like I said though "I don't really remember anyone rushing to Trillian because it was the best XMPP client it was because it could speak to multiple closed networks.".

I guess you could throw IRC in as a non XMPP open protocol but, ignoring that these were normally just gatewayed, it doesn't explain either the success or decline of Trillian. All of the supported 3rd party chat clients do.

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Since the internet became consumer towards the end of the 90s there hasn't been a time the majority of people were on open protocol chat networks or a multiprotocol client got popular without support for popular closed protocols of the day. Many clients and bridges have come but they have all eventually run out of steam trying to catch up to the latest hostile changes or the latest proprietary protocol that is taking off while none have ever supported more than the basic feature set like plain text and maybe images across multiple platforms reliably. Not saying it's impossible or people should be using closed protocols just that's how it's been.

To say closed protocols and clients are a new thing that killed the old multiprotocol apps is false, they started to solve exactly that probably but just ran out of steam. Now Matrix has some of us nerdy folks exited about an easy way to glue such networks together via an open federated protocol (which XMPP folks never really liked doing for some reason) and 3rd party integrations are being maintained again. They are still going to run into hostility and lack of feature parity but as we know from the ~2000-~2010 era it can work okay for plain text if you're fine with the occasional missed message or temporary protocol outages from upstream changes.