I'm not a Slack shill, have never used the product, don't know anyone who works for the company.
But.
Has Facebook ever been hacked to this degree? Has Google ever been hacked to this degree? Has any sufficiently popular social-media/enterprise-knowledge-sharing app been hacked to this degree?
What makes you so completely certain that they will be hacked and that a 'treasure trove' will be available to the supposed hackers?
Seems like unwarranted FUD tbh.