I still can't quite grasp these acquisition numbers. At that price you expect Slack to generate 1.5-2 billion in profit per year. That's 4 times their current revenue. When you factor in risk and time value of money it becomes more like 3+ billion per year. I just can't imagine a chat app driving that level of profit.
As someone who pays for Slack, it would be VERY hard to make a switch. The impact of this in retention and opportunity to monetize the 'integrate workflows' increases the multiplier. Salesforce has probably calculated how this opens up revenue to their existing product line and Slack itself is growing.
I'm not saying this is science but is part of the rationale that goes into the price.
IF your company is paying to preserve it. At my not-very-tiny employer we only pay for something like 1 year of retention. I have no idea if that's a legal or financial decision, but either way it seems like a dumb one.
Salesforce is betting that Salesforce + Slack is worth more than the aquisition price. And more than the cost of letting a competitor acquire it.
Combining all of these things into one offering is very difficult, and highly valued by both customers and the stock market.