That's a pretty ridiculous way of putting it, given that those same two people are also responsible for creating one of the most successful shows in TV history.
What will determine the success of this decision is the quality of what they produce. The number of people who will cancel their subscription simply due to D&D's involvement seems like a completely insignificant risk.
It was a bad PR move, and it makes it easier for spiteful fans to cancel their subscriptions. Normally just one of these things wouldn't be a big deal, but it seems like netflix has had a bunch of these "a bunch of your customers are so mad that they're leaving" events back to back.
Other than that, I think you are vastly overestimating the number of people who respond negatively to this, their actual responses, and their overall impact on Netflix' bottom line. The main risk here for Netflix is that D&D will produce a bad show, not that a bunch of teenagers from r/gameofthrones will cancel their subscription.