I think that what is happening can be described thus: waves of ignorance.
Every few years, generations of technologists evolve from their schooling, and come out into the workforce - competent, yes, but aware, no. Awareness of previous technologies is the problem: if you've never actually used e-mail productively, you may not be aware that people use it extremely productively, so you pitch an alternative - i.e. re-invent/NIH some 'replacement' technology that has a lot of fancy bells and whistles, but ends up re-implementing the stuff that your grandaddy programmers solved, decades ago.
Slack is an example of this - I see nothing in it that I can't do, effectively, with a well-run mailing list and well-configured mail applications, with company policies and, most of all TRAINING of employees/team-members on how to use it properly. The same goes for IRC. IRC and Email already solved the problems of Slack: its just that the mechanism (i.e. understanding) of applying those technologies is not evenly distributed.
It comes in waves, and I think it can be traced back to the timing of graduation schedules across academia.