Does it? My feeling is that users use whatever you give them. And then they buy a new device because theirs is "becoming slow". They don't say "wait, Slack is just a glorified IRC, it should work on my iPhone from 2008, I will just refuse to use it" because they need it.

There are no alternatives to the Slack ElectronJS Desktop app. That's the whole point: they don't want to maintain native apps. And they don't open their API to let others do it, I guess because that may hurt their lock-in benefits.