One thing to consider about Allegiant is that people above a certain seniority level rarely leave until retirement. So even if you upgrade early, you could find yourself on reserve and poor QOL for a long time. As said above, many of our senior FOs make more than the junior CAs and have a rock star QOL. It's common for these guys to credit 100 hours with 17 days off in a base with good schedules (long legs and lots of premium trips). IWA comes to mind.
I guess your bottom line should be whether you want to make big money over the course of your career at a legacy but be gone all the time, or make ok money and be home every night (for now… there is no guarantee we will stay that way). If you're young (below 40) it might be batter to choose the flow to AA because you will make millions more over the course of your career. Plus, a senior wide body FO at a legacy can easily make AAY CA pay. With room to grow. But if you're sick of travel, or have a family, this might be a good choice.