I used shouldn't in a negative point - making much the same point you are with it, so I think we're good on that one.
As for the level of effort based on statistics, that's an academic choice beyond a certain point. If I decide there's a low chance of having a physical attack, and only put in 10% of the training effort required to be very good, I likely wasted that 10%. That's my point about long-term training's benefit. I could quite easily teach an about-average person to defend against the most likely kinds of attacks in just a few weeks and have them be able to repeat those defenses under reasonable stress. A year later, however, they wouldn't be able to repeat those under the same stress - nor likely even under no stress at all. That's the nature of physical learning; it decays over time and will be reduced to near absence if it wasn't highly engrained. Now, if we're talking about the difference between choosing casual training (like my many years of mostly 2-5 classes a week) versus choosing a more intense regimen, I'm entirely with you on that being a selection based on likelihood of an attack.
And that same goes for the usefulness of physical training, at all. There are a lot of factors involved - too many for any level of certainty. Everyone I know who practices martial arts cites it as a major source of their personal confidence and feeling of control. Confidence is a determiner in how we act, and confident actions tend to lead away from victimization, both in target selection and in avoidance of circumstances. Add the locus of control, and the effects become intensified. So, now we've likely reduced the likelihood of being attacked, but of course not to zero, so the actual physical skills are to help with that remaining chance.
Of course, if the self-defense training is done right, it should also include some education on risk avoidance, threat awareness, etc. And those will have at least as great an effect on the chances of being a victim as the confidence gained will. I'm not aware of any study that has been done to try to measure that - nor am I confident a reliable study could be conducted - but I'd expect this last part to be the more effective part of the training, if we can measure a straight reduction in incidents.
Beyond that, of course, is the fact that folks enjoy their training. That's obviously a large part of why most of us train, though it has no direct bearing on self-defense.