I think you are really underestimating the bureaucratic overheads in a large company. It is not the same as a 3-person startup serving 5000 customers. Of course I pulled that number out of the air, but the point is that it can make sense. I think I am right around the ballpark of needing a 5 people team to serve 500 users - you'd need a minimum of 3 people just to make sure there is enough redundancy if one person is sick or goes on vacation. You'd need 24x7 on-calls. The scaling factor probably changes after that - may be it is 10 people for 5000 users instead of 50.

Adding to other large company problems, these people will need management layers, the team will need a charter for growth. Who wants to be the maintainer of the Mattermost servers in a large company? Add all of this up and then the slack deal starts to look reasonable.

Edit: Just to add some real numbers, slack costs $12.50 per user per month [1] - that is ~$750k per year for 5000 users = 5 engineers. (More like 3 really in the Bay area)

[1] https://slack.com/pricing