FR is a lot, the $10 earbuds would have atrocious FR. Either non existent or bloated AF bass with ridiculous peaky treble that you can't do anything to fix. Soundstage and some other things aren't modifiable at all really typically has to do with things like physical driver orientation in the housing etc.

I'm a huge fan of EQ. At this point it's readily available in things like the Qudelix5K for a very reasonable price allowing you to PEQ all the things. Why wouldn't one opt for making their headphones sound better given the chance. With auto EQ there's even a reasonably easy means of going to a neutral signature and making modifications from there.

That being said not every driver is created equal. Just because you've made modifications in an EQ does not mean the driver will be up to the task. Lower end cheap drivers will be too shitty to EQ and "fix" it'll just bleet and distort. Also just because a headphone is expensive does not necessarily mean it will EQ well.

Some headphones like Planar IEM's are well known for their ability to respond to EQ.