If writing a bot is a reasonable solution for IRCs shortcomings, then writing a Slack bot to do long term archiving of messages is also reasonable .

Slack's ToS specifically prevents doing that. Which, is reasonable, as if that's what they're selling, how would they make any money?

I'm no fan of Slack, but it has IRC and XMPP gateways. You can trivially set up an IRC bot that logs your channels & conversations, I connect to Slack via irssi & use its standard logging facility.

So you're no more likely to lose your logs via Slack than via IRC or XMPP if you set things up correctly.

So what you're saying is, Slack works great as long as you're willing to perform some 'uber geekery'? ;)

How do you think open source projects have preserved message history on the existing open source alternatives discussed in this thread (IRC & Jabber)? You set up & maintain your own logging bots.

I don't know if I'd consider that 'uber geekery'... it's just setting up an IRC client - they provide instructions on the slack website.