40%? Show me, please. I think it's more like 17%. You also fail to make note of the fact that most communicants today no longer believe in the Real Presence. And more people have the attitude that sin is no big deal. The consequence is a higher ratio of communicants at Mass--but a lower number of overall attendees.
A recent Gallup poll showed 50%, actually. With certain Protestant groups, it is fairly easy to discern the level of poll inflation caused by self-reporters fibbing, so I think other estimates I've heard cited by dioceses of 40% sound much more plausible, and in line with other polling.
Under-30 Catholic attendance is closer to your initial 20-something percent. Other church groups have seen these people return to church-going when they had children, but as other articles pointed out, there is some problem with that expectation.
Actually, I notice, as a general trend, where the Catholic population is youngest, attendance seems to be highest, so I have been optimistic of a reversal.
There are also those who imagine that schism and excommunication are no big deal. Go figure!