The problem you face is that decentralization per se brings very little benefit to the table and a lot of baggage that you need to evaluate honestly. If this pitch was to an existing organization like an enterprise the simplest question you are going to face and the hardest one for you to answer is simply "why should be take a risk on a (relatively) unknown and (certainly) more complicated solution when Slack gets the job done?" Unfortunately, I do not think there is a good answer to this question.
I really do not think that anyone is going to reject Matrix as a solution because ISIS used them, they will have a whole host of other perfectly legitimate reasons to reject Matrix. It only makes sense to take on the additional cost burdens of a decentralized solution if your organizational needs require it, otherwise it is a waste of time and you run the risk that your effort becomes one of those shitshow horror stories that turns into someone else's reason not to use a decentralized solution.
I really do not think that anyone is going to reject Matrix as a solution because ISIS used them, they will have a whole host of other perfectly legitimate reasons to reject Matrix. It only makes sense to take on the additional cost burdens of a decentralized solution if your organizational needs require it, otherwise it is a waste of time and you run the risk that your effort becomes one of those shitshow horror stories that turns into someone else's reason not to use a decentralized solution.