It sounds like you think I'm victim-blaming here and that's not my intent at all.

Part of being in business is anticipating risks and having a plan -- which could be deciding to accept the risk. What sucks is you're implicitly accepting the risk of anything you didn't think of, even if the seller is quite aware or even counting on it. It's a harsh lesson when something this happens.

Slack are leveraging their position and it makes them assholes (or capitalists, I suppose, depending on your point of view), but you can't control what they do. You can only control your choices.