Most teams in the NBA do not look to PG for a source of defense. In fact, there are only a handful of defenders at point that could be considered "good" defenders. Paul, Rondo, Wall, Westbrook, Conley, and maybe Curry (he's been playing well but is on a BEAST of a defensive team and hasn't been doing it for long enough to know for sure).
And keep in mind: We're in the golden age of PGs right now. The point is just historically not a point of strength for defenses, so there are a lot of teams that could theoretically improve their defenses without a point, but it would crush their offensive production AND generate a lot of easy bucket turnovers if they don't have anyone to handle the ball up the court.
Patty Mills is not a viable solution at this point. I can't believe anyone would think he's viable right now when he has been far and away worse when he's handling the ball than Parker. It's baffling. Then again, it's SpursTalk, we're 5 games into the season, so I suppose I should expect a meltdown even if we were 5-0.