As a dev I'm biased of course and this might not matter to most people. One thing though, you don't have to write the full app three times. For example all the business logic can be in C++ and write a native UI on top of it.
I think it might matter to non-devs but the tradeoff has to be pretty compelling and in Slack's case I don't see it. Like, there's still dev work happening to make these apps, so it's not like you're shutting out a whole group just because it's a web stack and not C++.
As a dev I'm biased of course and this might not matter to most people. One thing though, you don't have to write the full app three times. For example all the business logic can be in C++ and write a native UI on top of it.