You're right about the IM apps being not great. This and the poor state of desktop email clients on Windows and Linux are my pet hates
I've had facebook messenger, gchat/hangouts installed on my phone for some time. I don't know if I'd call it great, but I have a hard time arguing that it doesn't immediately solve my instant messaging needs.
What exactly do you want that gchat or facebook messenger doesnt solve?
Whenever I’m stuck using a centralized service (Twitter, facebook, some web forum, whatever bullshit IRC replacement businesses are buying this week, ...) their client invariably does something different than what many people want it to do, but we have no recourse because everything is locked-down proprietary bullshit, even though most of the messages are plain text and could have been delivered just fine by open protocols in 1990. Not to mention these apps are all poorly designed resource hogs that spy on everyone, inject ugly ads, and burn mobile battery.
1. gchat (or "Google Talk" I think?) was a nice simple app that was replaced by a rebranded one that had different functionality and a UI that was either different or flakey/slow/unreliable (I forget all the details). I later learned that actually we were all duped by google - gchat itself was only turned off completely very recently, but at the time they managed to convince us that we had to use Hangouts instead. In any case, the whole thing was annoying enough that we left because ...
2. Whatsapp was starting to be very widespread and was a natural replacement for us as most people either had it or could quickly get it (you just needed a phone number and nearly any sort of phone, many feature phones support it) and it had a simple chat-only interface.
3. FB Messenger was not a separate application at the time, it was integrated in the FB app iirc and you still had to be a member of FB and add any contacts as "friends" to communicate. At the time it wasn't seen as a general-purpose chat app by _anyone_, just a way to talk to your FB contacts.
Hope that makes sense. If you're curious about the gchat->hangouts switch, I'm sure a quick search will turn up a few disgruntled articles - there were quite a few people who were annoyed. It probably didn't help that Google were in the habit of dropping products, so a few people may have assumed that they'd do the same with the chat app.
I realize there are per-thread mute settings, but the whole experience of using these things is highly intrusive. At least with text messaging (or calling) we still have, to some extent, the social expectation that it is known to be intrusive, so people (in my social group at least) are less likely to text for frivolous reasons.
Notifications from people are fine, e.g. 'x messaged you'.
Notifications from machines are the problem, e.g. 'x just did y'.