You can make $150k/year at a legacy as an FO with the same longevity. You'll also have a 15-16% direct contribution plan that allows for better retirement while at the same time allowing you to keep your own monthly earnings instead of spending it to receive a meager 401k matching.

I'd equate what's going on at Frontier and Spirit to what we saw in the mid-2000s at regionals. Fast growth and folks incorrectly correlating a quick upgrade with QOL and high compensation. Obviously a fast upgrade is nice, but at a ULCC it's a necessity because the FO pay is so terrible and there really aren't any other compensation package benefits worth mentioning. At a legacy a quick upgrade is nice too, but upgrade is not a necessity to achieve a high income and retirement. Therein lies the difference.

On the other hand... The big question is how long will the pay-leaders of the industry go before their next house-cleaning. It's a crappy industry and you're rolling the dice anywhere you go.