Terms and conditions have not suffered in the airlines you referred to because of a lack of a seniority. The conditions have eroded due to a lack of union representation
Union membership is fine as long as long as it not compulsory to join and you have level headed commonsense people running it. Because of the nature of trade unionism, historically it is the loud-mouth heavyweights that get to the top of the union executive and the rot starts from there.
The Australian Pilots's strike of 1989 is an example of this. 1500 pilots were given bad advice by the highly militant union heavies and lost their jobs for good. The problem in Australia at the time was it was compulsory to join the pilots union otherwise no job. Competely illegal of course but the government was also cowed by the militants and looked the other way. Union representation might get you better pay and conditions initially but that's not much good when your job is later lost because of union militancy..