Is it possible to sprain the brain? I’m asking for the small minority of Cardinals fans who are overstretching themselves by trying to find potential downsides to their favorite team trading for superstar third baseman Nolan Arenado. Most in Cardinal Nation are floating, for good reason. But a few are fretting, worrying about what-ifs as the Cardinals work to finalize the blockbuster deal. Let’s examine their concerns. What if Arenado winds up using an opt-out to depart sooner rather than later? Don’t bet on it. But I’ll play along. Let’s say Arenado turns out to be a short-timer in St. Louis….