Yes, volunteers ran these things. How many? Because you're going to need to have paid employees rather than volunteers running your servers. Is it going to be more reliable and cost effective than just paying a third party to run a service for you?

If you're a big enough company (more than 60 people if computer are your means of production, around a thousand if not), you should have your own server and a fallback on a CSP. Just pay some guy in India/South Africa to manage all your internal services and architecture. If they are sensible and you need to trust the guy, just pay him well and hire locally.

I can set you up a resillient architecture and 3 services (communication, documentation, filesharing) in less than two weeks (let's say two weeks and a half because security concerns/limitations and all that stuff). Add one week and you will have mail server, VOIP communication, log conservation and monitoring, backup images on CEPH, vnc service for "easy" maintainance and a small documentation explaining the few action needed to maintain the monster for two years. Initial cost for a company: 15 to 25k$, + 1 hour and ~100$ a month.

For something like IRC? Yes. The service is dead simple and one server can handle thousands of users with modest hardware and without breaking a sweat. A completely unfamiliar sysadmin could set it up in an hour or less. Since it's probably internal, there's little need for public services (nick/channel registration, etc).

So what about the mobile device issue? Corporate logins?

If you want to match what Slack offers you aren't just running an IRC server, you're going to have a small fleet of dependent services that, added up, just about match what Slack does.

Is it really that surprising that people just opt to pay for someone to deal with all of this for them?

Put TheLounge [1] in front of it for people that want a web interface. Perhaps use ldap for corp users. [2]

[1] - https://thelounge.chat/

[2] - https://thelounge.chat/docs/configuration#ldap-support

At which point you're not looking at IRC as a service anymore. There are different niches served - but the question was about the reasonableness of setting up IRC alone, which is still dead easy.

Most of how I use Slack, and how everyone I know uses Slack, involves 1. sending messages to people who are “away” to get their attention/loop them into a conversation; and 2. people reading back up in history for context when they come online.

Now, modern IRCds might have features I’ve never experienced on raw IRC... but if they don’t, I don’t suppose you’re suggesting that this server also run thousands of bouncer bots in order to provide this contextual history feature?

(Also, whence push notifications?)