There were dozens of terror attacks against air travel in the 80s and 90s. The global post-9/11 "security theater" did make flying in the developing world much safer.
To be fair there's been a lot less hijackings in the 2000s and 2010s compared with previous decades - although as the decline started in the 90's - its not likely to be solely due to increased security.
It has to do with several things, among them you have:
- the end of the cold war, with the clear hegemony of the US which halted state-sponsored terrorism from unaligned-but-socialist-minded countries (especially Libya)
- targeted assassination of terrorist leaders and infrastructures (no more training summer camps) no matter the country they are in (mostly through drones nowadays), leading to a progressive reduction in sophistication in terror attacks committed, and the rise of lone wolfs instead of structured terrorist commandos.
Grammatically, 1.5 donut is wrong and 1.5 donuts is correct. So, in this case "1.66~ dozens" (because 20 / 12 = 1.66~) is pedantically correct.
(...But logically and intuitively everything I just said is extremely stupid because by that logic 12.01 counts as "dozens" and that's ridiculous, so your complaint is absolutely correct and I agree with you 100%)
Wasn't that a contributing factor to UA93 being "unsuccessful"? The passengers had gotten word of the other three flights, and stopped being quite as compliant (though "compliant" is obviously not the full story for the other three flights).