Based on conversations with folks I know who came up in that era, it was MUCH harder to get that first commuter airline job. I think there just weren't many available in the pre-RJ era. Guys talk about needing 3500+ with lot's of twin and 135 time to get a commuter job.
But once you got into commuters I think the next step to the majors was easier, since there were lots of mainline planes and only a few t-props.
Today it's the other way around...300 hours to get that RJ job, but then you are probably stuck there for life unless you know someone.