I agree, when my company moved from self hosted mattermost to slack, it was a definite productivity boost.
Because for us it was different. We got rid of Slack and decided to just use basic Google Chat we already had with Google mail. It was definitely a productivity boost and insanely decreased the number of times people messages each other. It was helpful.
So what, in your case, it had to with self hosting? Or did you mean it was just the magical effect of Slack being Slack?
Slack has a lot more product thought and nice features that keep on getting added. Our competency is not managing mattermost, it's doing what the company is made to do.
If you want to reduce communication, well that is a different thing altogether and you can choose if you want chatroom software for your company or not.
The thing though; is that this argument can be used to outsource everything.
Office space, HR, recruitment, even making coffee.
At some point the externality puts restrictions on your maneuverability, in the case of office space it's because you might not be able to just run a few power cables somewhere, for HR (imagining a scenario where it's outsourced) there's limitations on speed, or in the worst cases limitations on how you can even interface with the HR department.
Making coffee is one of the cheapest things you can do, outsourcing it may mean, though, that you lose choice of what beans you have, or what types of milks you want to stock, and there's a premium on the price because another company wants to make a profit supplying this service.
This is a contrived example, but overall my point is: there are times that in-sourcing your tools actually gives you the time and freedom later. It's a gamble you make.
We'd love to improve, are there one or two things top of mind for us to change? You mentioned occasionally missing notifications?
Once recent new feature slack added that I like was scheduled messages, but that is a pretty new thing.