I'll be honest, I think you've got an uphill battle to convince so many of us that are concerned about accidental pricing that this is exactly the thing we actually want, vs. what we feel we want. This position parrots what Vercel's leadership said when someone had a massive surprise bill, and the story made the rounds both here and elsewhere.
To be super honest, you might be right too. You might go down a huge engineering effort to build this in only for 5% of your customers to ever engage it. I think the real question is what percentage of your customers will feel better knowing that the have the choice to set those limits, and how will that comfort actually improve their trust with Fly, and cause them to choose it over another cloud provider.
It may end up being a lot like this feature we had in a platform that I used to support. It had a just-in-time analytics pipeline that at one point required tens of thousands of dollars in compute, storage and network hardware alone to function. Based on our analytics, it was barely used compared to the usage of the rest of our app, which made the zounds of resources and fairly frequent support attention it needed feel silly in comparison, so I advocated to sunset it. Product assured me that, regardless of how silly it might be to continue supporting this feature, it was a dealmaker, and losing it would be a dealbreaker.
So yeah, y'all might be right in that the majority of your customers don't actually want it. But maybe what they do need is to know that it's there, ready for them if they ever need to engage with it.