I feel like you're living in a different universe then. I will literally never fly Spirit (well, neither will anyone else) nor Frontier ever, I loath the experiences I've had on them so much.

First, as someone with relatively long thighs, I literally don't fit in their sardine can seats. But more relevant to most people, while things may be OK if everything goes perfectly and nothing is delayed or cancelled, you are completely SOL with Spirit/Frontier if something goes wrong (and "something" may just be they themselves decide to cancel an undersold flight at the last minute). It's nearly impossible to get someone to talk to, I feel like the employees know how shitty their companies are so they all have an attitude like they DGAF, and it's a mad (expensive) scramble to find alternative arrangements at the last minute.

I've never had as abysmal experiences as I've had on Frontier compared to any other airline.

From a customers' immediate point of view, this sucks for you.

But it's great they are not regulated utilities. Because either everyone would have to pay for extra legroom, even if they don't need it, or some freakishly long people would not be able to pay for the extra legroom that they need.

I’m relatively tall and have a generally rough (but tolerable) time with all domestic bottom-tier seats.

I have no difficulty believing you when it comes to customer service. I’ve never had any issues requiring anything beyond the most basic customer service, so I just haven’t been exposed to differences between airlines in that regard. I also understand that a bad experience can leave an exceptionally bad impression. I suppose the only thing that might surprise me is if the higher-cost airlines don’t also have terrible service.

Yup, came here to say this. Once you're on the plane and its in the air, Spirit and Frontier are like pretty much every other domestic airline. There's slight variation in terms of whether you get a whole can of coke for free or not. If you're taller than me, the 28" of seat pitch vs say 31" on delta may make a difference, but I'm only 5'9".

I still avoided them like the plague because the legacy carriers are selling you operational performance and the ability to usually get you where you're going within a reasonable timeframe if you're delayed or canceled. Spirit, Frontier, Allegiant, whoever else, do not do nearly as good a job when something goes wrong. Although they should get a lot of credit - none of them have ever had a fatal crash.