[Miller]

This thread certainly went off the rails from an outside view, the main issue with Spirit Mirror is that denying the other player any means of knowing it's there or not completely defeats most of the point of counter play and heavily skews a battles' favor towards whoever ran into it without being aware.

Could you argue that Mysterious should be valuable for these kind of scenarios? Absolutely, although I personally agree on the notion that Spirit Mirror's effect is really powerful for what it does. But part of the issue is when these sort of information denying mechanics are so common place (Mask of Metal) that it becomes frustrating and obnoxious in most scenarios that they occur, especially when Spirit Mirror effectively denies you of 3M (Can be more or less) and potentially damage you in the process of doing so.

"Why not just null or throw the funny rock at it?"
The former is likely due to a lack of good sources and some of the best sources of said nullification already running into the spirit mirror before their null actually goes off (Null Shell); the latter is because throwing Requinite blindly into someone who /MIGHT/ have Spirit Mirror isn't practical.

The whole point is information denial making the item more obnoxious and having the main counter-play be 'I should check if they have a spirit mirror before I use x' puts you at a momentum deficit over your opponent for the cost of a singular accessory slot.

As for Tana's post, I could potentially see Spirit Mirror getting a change that allows it to refresh at some point, but I personally feel like it's current effect compared to most other accessories is already a league ahead of what they have to offer. Maybe a 10* upgrade in the future or something could have something along those lines.