Most people use whatever is dominant. People who have contacts on more than one 'app' simply install those apps and mope about it a bit (but accept it nontheless). In the Netherlands (and a lot of other countries), the dominant player is Whatsapp, although some countries have their own dominant player, such as Kakao Talk in South Korea.

Of course there is call for unified messaging; it just isn't in the interest of the companies behind the currently dominant messing apps to facilitate it. To monetize their product, they need you to use their software, on (or through) an operating system approved by them, following their rules (e.g., Whatsapp's requirement of a relatively high-value personal identifier in the form a phone number). Using anything else to access their protocol causes a devaluation of their product — whatever their eventual business model will be after the make-sure-everyone-uses-us phase will be (advertising, user tracking/marketing profiles, freemium model), users will need to interact with them on their terms, with their software.

That was always the case, though. The problem with making a WhatsApp Pidgin adapter, for example, is that WhatsApp only allows a single client. I'm not sure why nobody has tried to make one that talks to the phone, like WhatsApp Web does (although that also only allows a single browser).

Generally, we've taken a big step backwards where chat is concerned.

I think federation has largely moved to the device. I do most of my messaging on my phone, where which app I'm using isn't invisible, but it's also not really a hassle to switch (mostly it involves dragging down my notification shade and tapping on the message I received). Not perfect, but also not really requiring the cognitive overhead that switching between desktop clients seemed to have. My phone federates my contacts, email, calendars, etc without much input from me and I believe chat will go that direction as well.

Your phone federates your contacts, email, calendars, etc because those are all open protocols.