No Flak From Here!
This may seem more common bit it is not; we simply HEAR more about it these days. IMO, ground the idiot and get him into treatment. If the Tx takes, monitor him for an entire year and then - Only Then - reconsider his license status, perhaps requiring him to fly as a supervised F/O for yet another year, before reinstating that fourth stripe.
And YES! Someone should have taken action long before this fellow made it to the flight deck. Better to test - and be wrong - than to not test and be even more wrong! Ninety nine SLC's plus crew?
You are correct that 100% pre-flight testing cannot be done. Random testing should increase - a lot.