The reason there's competition is because ~"let's only field one perfect option" sounds great if you think you're the one with the perfect product, or you're a user whose needs are exactly like that product's pitch. In reality, user needs are diverse: there's no singular perfect product.
In this particular case though, note the title of the article. Wouldn't it be much more convincing as "A Superior Alternative to Slack (Which Happens to be Open Source)"?
EDIT: You're second point definitely does apply here though.
All of this is a response mostly to buzz. Slack looks great, and is absolutely popular. So we (the open source community) want something "equivalently buzzworthy" while forgetting that we are, in fact, the entrenched monopoly already.
I'm not losing much sleep, nor will I be retiring my IRC client or pastebin links any time soon.