I don't know… If I'm sitting at home or at a cafe working, I want my headphones to block all bicycle bells and ambulances on the street. Those in traffic could perhaps just turn their ANC off?
And of course there will always be fringe cases. What if I go to sleep with regular foam earplugs, what if I take a sleeping pill etc. Or what if the warning sound can't be engineered to fit a ANC friendly frequency, like somebody screaming, a car tire screech behind me and so on.