It's not just the setup of a self-hosted solution, but maintenance, and otherwise. If you look at the enterprise space, bigger and bigger companies are becoming comfortable with cloud-hosting of services. Growth of companies like Okta demonstrate that shift. As far as it being acceptable, I think there are a few things to consider:
1. The Slack model is such that your staff could start using it without even getting permission from the top. This is the Slack strategy for sales. It comes into companies from the bottom, so companies are more responding to the fact that their employees are using it vs bringing it in from the top.
2. Yes there are risks with cloud products, but risk is a cost consideration so you look at cost impact to the company of a breach and you compare that to a self-hosted high maintenance solution. This is a much more difficult calculation and it really depends on the size of your company, the value of the information Slack will be holding, etc. It's also possible Slack could be seen as more secure because an internal system breach may not include a complete Slack hosted breach. It could be seen as data segregation and diversification.
3. Slack is not the only company that is making inroads here. Slack is known well in the tech industry, but less-so in other industries. Microsoft is a giant because Skype for Business is huge, and there's many others.