It is clear the economic, social, and safety impacts of selective, tactical removal of signal are considered
"Selective Availability" is just that - "selective". What happens if the US has a bit of biffo with some of our northerly neighbours? They can decide to deny them (and thus us!) availability of GPS, while everything is hunky-dory in the Americas and Europe. Considered and actioned.
I know officially new satellites are launched without the capability - but that doesn't mean some new software can not be uploaded at any time.
There is one major issue with turning on SA in this day and age, though: they can only do it once. It will take experts only a few days to crack it and the enemy front-line troops will have Android phones with perfectly operating GPS in weeks. The next line of consumer and aviation GPSs will have upgradable firmware to get updates for the hacks.
In any case, that's why we should support Galileo and have dual receivers. Push come to shove, some government controls that too, but the more systems we have, the less likely it is they all go down.