I've personally built all of the things you're talking about, on real products, with lots of traffic, and it just isn't that hard. Annoying, yes, but arguably a core competency of any data team, and something you want to think very carefully about outsourcing. Also, most teams won't have to "scale" many of the things you're talking about...that's a problem that you only have to solve when you're the central point of failure for lots of companies.

As for the n^2 problem, yes, I can see that argument, but in my experience, n is always a small number (i.e. 2-10), and they rarely need to cross-communicate in a fully connected graph. In short, piping everything through an expensive, centralized point-of-failure run by strangers seems like such a risky proposition that I can't imagine many companies would have the integration complexity to justify the risks.

But again, people seem to be finding value in it, so maybe I'm missing something. I was honestly expecting you to tell me something about advanced data warehousing or integrated analytics tools or something like that. If you tell me that integrating with segment automatically starts correlating session data across providers, well...that starts to be compelling.