What would it take to make you unsub from xiv

[Eisi]

It isn't even clear what a non-vessel Shiva would look like, is it?

True, and there shouldn't BE a non-vessel Shiva, not because there technically couldn't be, but for the same reason why we needed a vessel for her on the First - because Shiva in FFXIV = vessel. It's an opportunity for XIV the be unique and if that is discarded then I believe it's because it was either forgotten, aka they stopped caring or it was forgotten how meaningful it is, aka they stopped caring.

Also, imagine how flaccid Eden's excuse would feel if you play it as a Summoner who learned to autonomously summon Shiva on their own.

Saying that, I don't think its an impossible ask - I mean, our character can turn into a Phoenix 'mount' - could that be applied to battle instances?

Shiva has always been summoned in battle instances so yeah, 100%. More than just not an impossible ask I think it's neccessary for her to function like that.

The Phoenix "transformation" is a dumb meta reward for an event and let's be real - it's not a transformation, it's just a shitty moneysaver mount they didn't need to put in the work with to have the WoL actually sit on it. A transformation mount wouldn't have the classic horn blow to summon it, but instead a transformation sequence which they didn't add. Cause it would have cost them. So at the end of the day it's an unusable mount for people who like mounts and people who like transformations both, cause they didn't care putting in the work. They should have never released it in my opinion.