No matter how smart or skilled they are, people who spend an inordinate amount of time arguing politics (and dragging others into it) at work are by definition not "the best staff".
Having a narrow mission focus is the only way to retain your actual best staff.
Not necessarily the case. Consider the possibility that strongly-held political beliefs correlate with software engineering skill, in which case it could easily be that a politically-active but superior engineer comes out ahead despite "wasting" some time on political issues. Yes, they're not as productive as a hypothetical engineer with the same skillset and no political beliefs - but that hypothetical engineer may not exist. Alternatively, consider that forty hours of dedicated software engineering may be beyond the capability of the human brain and that political arguments happen during what would be classified as rest and relaxation time if not for labor standards developed for physical laborers and factory workers in the 1930s. Alternatively, consider that most people work better with motivation, the belief that you are doing good is among the healthiest and most powerful motivators available, that maintaining such a belief requires continuous effort, that such effort would likely involve discussion, and that that discussion would almost necessarily happen at work. I'm not claiming that any of these scenarios are the case. I'm simply pointing out that they are possible and plausible, and thus it is not necessarily the case that an employee arguing politics at work is underperforming, either in relation to their own potential or relative to others.
Having a narrow mission focus is the only way to retain your actual best staff.