There are many ways in which I prefer IRC to Slack, and I agree with Sir_Cmpwn's general premise that some of Slack's features are more trouble than they're worth. (Slack's threaded replies are definitely one of them, and while I've seen better implementations in IRC-like systems, "just don't thread replies" is so far the best implementation I've seen.) But the problem you're describing here is arguably one of Slack's advantages, not a disadvantage.

Are you living in a world without netsplits?

Netsplits are pretty rare these days.

Yeah? I just counted six netsplits in the past 24 hours that affected a channel I'm on on IRCnet.

I still love IRC. My second home.

This is because of IRCnet is operated by multiple entities and has rather peculiar links, other networks are not like this.

http://dx.fi/alt/ircmap/fdp.png here’s a map of IRCnet, they’re really going out of their way to generate netsplits.

A network operated by a single entity doesn’t need such a silly amount of nodes, you can support hundreds of thousands of IRC uses on a single server.