Your assumption of 1 hour of productivity lost daily does not consider how much productivity is freed up by DM conversations taking place in chat rather than walking around or ‘watercooler’ talks. I don’t question your numbers but in our company, post Slack adoption, we’ve seen an increase in productivity and, more important, much higher awareness of what other people actually do. In a rapidly growing company, this is a critical issue. I think, rather than open with how great Slack is, and then eviscerate it, you should do the same evaluation from the positive productivity and cultural POV.
BTW, I am a 61 year old guy in a fast moving software startup (not my first) so I go back to the old days and chat is a vast communications improvement, IMO.
Thanks for the thought-provoking piece how on to make data tell you what you want it to! Seriously.