I hope I'm wrong, but I think you're missing the point here. Users may like having separate tools to help them differentiate between different communication tasks. I don't think that trying to manage a bunch of messaging apps is a pain point for most/all people.

It is because the user ends up having to look at notifications of multiple applications. Those notifications take up space.

Right now they're sorted in a way. For example, the way the lockscreen looks, or the time they've arrived. This doesn't necessarily reflect priority for the user. How to achieve this? I think the correct way is by ML/AI. One way could be looking at which notifications the user tends to open first. Or, for example, look at which correspondence appears to be most used. This could even be combined with other factors such as time of the day, day of the week. E.g. the wife might be less important during 9-5 at a weekday.

Moreover, to reply to your specific point, a user could decide to user Ulum, plus Signal on the side. Or a user could decide decide to run Ulum, MSN Messenger, and ICQ. My point being, an application can abstract some of the protocols but just because it can't do more doesn't mean the user will use that feature. This is entirely up to the user.

Right, everything is entirely up to the user.

If it isn't a pain point already, then by definition you're solving a problem that isn't there.