The (usually unstated, often unrealized-by-the-proponent) assumption of the "9mm is now good enough not to fool with 40 or other more powerful calibers" is that bullet performance is binary - it is either "good enough" or "not good enough." If "good enough" is defined as the FBI protocols, and it's binary, then, yeah, you end up with 9mm as the solvesall answer.
But the binary thinking is pretty questionable. Let's apply it to another context: baseball. Instead of cartridges, let's evaluate baseball players.* Let's consider two first basemen from history: Lou Gehrig and Mark Grace (the former Cubs first baseman). During his career, everyone agreed Mark Grace was a very good player. His career batting average was over 300, he got MVP votes in at least 4 years, won 4 gold gloves, etc. There is no question that Mark Grace was "good enough" to be a MLB 1st basemen... and teams agreed so much they paid him MLB salaries for 16 years.
Lou Gehrig was, in virtually every measurable way, a better player. If you doubt it, you could look it up.
Yet the "good enough" binary assumptions of some would lead a baseball GM (with access to a time machine or a cloned copy of both players) to say, "well, Grace is likely to be cheaper, and he's good enough. Basically, all batters suck - they fail more than half the time to even reach base - so let's just get one that meets the basic standards and call it a day."
I find this a less-than-compelling line of reasoning.
* Like cartridges, sometimes even a not-very-good player will get the job done and get the timely hit or bonk one over the fence; conversely, even the best players strike out quite a bit. Similarly, sometimes a .22lr round takes out an assailant, even though we all agree that's not really a very good defensive round; and sometimes an assailant will somehow keep coming after being shot with a rifle (citations for high military decorations often include this as part of the decorate-ee's story).
