There's limited jobs with which a commercial licensed pilot can gain hours (e.g. crop dusting, sky jumpers, instructors, etc) but the number of people trying to attain ATP/jobs requiring ATPs well outstrip the number of jobs to support those people getting to 1500 hours.
As you said, previously some airlines would take the pressure off by hiring commercial pilots and getting them hours as copilots. With the new rules they need 1500 before day one, so that has to come from somewhere and it isn't clear where.
This is one reason airlines are struggling to find pilots.