My memory is fuzzy a bit, but I think I saw Slack being called, or even calling itself, the "e-mail killer". This leads me to believe that the hybridization you mention is on purpose, and its goal is to make profit by destroying the only remaining decentralized communication protocol in widespread use.
I'm pretty sure it was discussed as the "email killer" even here on HN, where there is a tech-savvy audience. You know, in one of those recurring waves of comments to the effect of "email is broken" (and its cousin, "it's time to move past textual representations of code"). Thankfully after all this time email is still used by devs and text remains the primary representation of code -- both for good reasons!
