This is only regarding full size over the ear headphones, since iem’s have their own price brackets
So I would classify mid fi into 2 categories. Upper mid fi and lower mid fi. I think that stuff under $150 USD would actually fall into consumer gear. Around the $200 mark up to the $350 mark would be lower mid fi. People who have graduated from more consumer oriented gear like the hd5 series (excluding the 58x), mx0s, sony’s, stuff like that which have a more “mainstream” tuning. Lower mid fi headphones usually are great bang for buck performance and cover what most average people would want out of a headphone, (headphone novice’s “endgame”). This is also the range that lots of experimenting happens to figure out what the person likes, and owning multiple headphones can make sense, since imo at this price range alot of headphones like to emphasize a certain aspect about them to attract new buyers interesting in a different sound, hence “mid fi hell.”
The $350 to $650 range is more upper mid fi, as it typically has a higher tier of performance, and gets closer to the luxury item territory, as the build quality usually reaches its peak ignoring the high end segment, and sound starts to get balanced and more usable for multiple purposes, and because of the price, most average people tend to stop here with one or two pairs. People usually experience this performance segment, and start to question their more lower mid fi cans, and sell them off for one or two upper mid fi cans. Diminishing returns really starts to show here, and thats why most do not decide to take the step into the higher end.
I think the term hi-fi is very convoluted and differs depending on the person, so I really can’t answer that (I actually wrote a categorization essay on this very topic, but can’t find what I named it unfortunately)
Edit: also, I do understand the leaps and bounds in the under $100 market, but I don’t feel that it would go into a category called mid fi, simply because of the idea that it should be in the middle. I also don’t think that the “consumer” level gear is bad at all, or should not be considered high end audio.