Nothing guarantees you a mainline job, not being hired at mainline (furloughs happen, people break stuff, training failures etc), and certainly not a flow. Ive flown with countless people who had a flow. So why are they still at 9e? Because flows can go away. You want a guarantee of a mainline job? Get a mainline number when hired at your regional. Anything less is just a pinky swear and bait to get people who are too naive to realize that these flows exist and then disappear without warning.

You are far better off working at a carrier with good work rules, good pay, and a base you want to be in and interview with the same airlines that are making people wait to flow or any other you are interested in than to listen to the guy saying everything will be great someday while you sit on your hands. If the airline you are flowing to wanted you to work there, they would have just hired you. They want you to work for their regional and the first opportunity to pull a flow that arises, it will disappear. You are much better earning money and having good QOL.