It prevents that specific strategy that would culminate in extinguishing. The idea being to siphon users away from other platforms, then add features that other platforms won't or can't implement, and use that to create an image of their own platform being better, having more features. If they succeed at having a lot of users oblivious to what's happening, they will use those features, and when they don't work for people on other platforms, they will blame the other platforms instead of their own, further cultivating the image that other platforms are broken/unreliable. In the end, they leave other platforms unable to compete, forcing users to either have a "broken"/incomplete experience, or migrate to their platforms. (Or leave the fediverse entirely). Or they can simply stop federating at that point, after users have left for their platform, cutting off the rest of the fediverse from content hosted on their platform.

The way defederating prevents a strategy like that is by cutting them off before they can get a foothold - they can't make users feel left out if they don't get to influence their experience in the first place.