It was quite safe in the 15 years before 2001 too. As for the major incident that happened in 2001 -- it wouldn't have been stopped by any amount of scanning of people's laptops.
Cockpits are locked. Passengers are more vigilant. Bruce Schneier argues that this made all the difference. I don't know if it's right but it seems like not a stretch.
If anything, the most (or only) effective aspect of TSA checks are that they remind people to be vigilant. That may be the single best reason to continue using security theater, and a very strong argument in favor of keeping it.