When I worked jobs with similar functions, the idea of expending effort organizing and agitating when I planned on being gone after a year would have been ludicrous. Unionizing is generally a short-term loss for workers under the hope for a long term gain.
As for whether or not the union can hold if scabs can be brought in...not generally. They can unionize, but their main source of power (withholding labor) is greatly muted. Even industrial unions, where time to productivity is probably higher, generally have to discourage (even at times to the point of violence) scabs during periods of heavy labor militancy.
That wage scale is carefully negotiated between the union and the company to divide the value created by lower labor turnover and increased specialization/experience. The cost of the union is relatively low compared to the value created by having well-trained workers. It is likely that equally skilled workers could attain the same (or better) pay working for an equally enlightened corporation, but that would be a rather unstable setup.
Hiring a few new employees and training them is easy for an employer to do, but being forced to suddenly rehire your entire workforce is daunting. Any business would suffer dramatically if forced into this situation.
There's always the possibility that your starting assumptions are wrong.