It's generally accepted that unionizing retail staff and other short-tenured labor (i.e., workers don't stay at the job for long periods of time) is very diffic

It's generally accepted that unionizing retail staff and other short-tenured labor (i.e., workers don't stay at the job for long periods of time) is very difficult. 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.