$80,0000 a year, for that sort of money you could hire an IRC developer full time and get them to spend a day or two managing the company server.
How can anyone find a decent developer for $80k? Even not factoring in overhead and benefit costs.
In most of the world that will buy you a decent mid level developer at least, a great senior or two in many. Even if it's below market, if this was my pet OSS project I'd happily take a pay cut to get more job satisfaction.
99.99% uptime means it can be down for a few minutes a month, so all it needs to do is fail over properly. In practice, it will probably have many more than 4 9’s.
I think the real reason slack does well is ease of client + service setup, the brain-dead UI, lots of feature creep that a few people care about, mobile clients, etc, etc.
I’m not a huge fan, but it could be worse. At least they didn’t leak everyone’s password like hipchat did.