The standard refrain in the 1970s and 1980s was that if your plane was hijacked (or other terrorist hostage situation), do not resist. The hijackers only wanted publicity, and if you didn't bother them, then you would eventually be released safely. It is certainly what I was told by my teachers and parents during news events of hijacked planes and such.
Such advice died on September 11, 2001.
So if there was a similar attack in 1975, it would radically change everyone's perception of what to do in a hijacking. Airport security would be dramatically strengthened, plane travelers would be told to do everything they can to not allow hijackers to take control of the plane, and special forces would be sent in to take back planes whenever possible just like it is now.
For all practical purposes, it would end the era of plane hijackings.
If the PLO was involved, it will kill any support for a Palestinian state in the US. A 1982 scenario where Israel invaded Lebanon, and the US assisted the PLO in leaving Lebanon for Tunis would never happen. US policy would be to support anyone who went after that group. It could very well lead the US to attack the PLO directly itself - even the Soviets would realize the US would need to massively retaliate against the perpetrators although they would not want to see the US expand reprisals to include other actors that were their clients. Likely some kind of informal arrangement would be made between the US and USSR that established what the US could safely do that would not trigger a Soviet response.
There are not many groups that would attempt such an attack in 1975. While Islamic radicalism was beginning to boil, it was minor enough that nobody thought it was important. Only until the Shah's overthrow and the Siege of Mecca in 1979 that Islamic extemism became known in the West. Terrorism groups were either political Marxist groups or secular Nationalist groups.
Afghanistan had not broken down yet by 1975. The king had been overthrown in 1973 and a republic instituted, but Daouad Khan was still in power. The communist coup and Soviet invasion had yet to happen, and Afghanistan was not a hotbed of Islamic extremists.
