What's the point with that quite lackluster story in Endwalker?

[Sicno]

It’s explicitly stated that the color never changes around everyone else.

I think I know the part you're referring to. Meteion says:

"Like me, they feel his pain and turn dark...
That's only for Hermes, though. For others, they're always white and bright."

I honestly don't take that too literally because I find it entirely unlikely that the only emotion ever manifested in the history of the ancients is Hermes's sorrow. It could just be poor phrasing or interpretation during localization. The intention here could have been that even when they change color they remain a bright hue, like blue or green, and that they only go dark for Hermes. In an earlier quest he says:

"Though be it here or elsewhere, they are seldom seen in any hue save purest white..."
"(...) While it has no will of its own, it is sensitive to the prevailing emotion in the vicinity, and reacts by altering its color and vibrancy."

He doesn't say "it reacts to sorrow". He speaks about prevailing emotions, which also checks with what we saw earlier through he MSQ, that it changes to other colors because of different emotions, not just sorrow. So it must have reacted to other emotions too for the ancients. I know this contradicts what I said earlier about literal interpretations after localization, but if they would indeed have only ever reacted to him we could expect a more binary representation. Black or white. Joy or sorrow. Not hint at a spectrum of colors nor to emotions in general.

The Elpis flowers respond to the emotions of others, it doesn’t effect change as it lacks a will to do so. Standing near an Elpis flower won’t make you susceptible to its influence through dynamis, but standing near Meteion? Different story.

The flowers are just an example. They say dynamis is a form of energy, so it follows that it could be used or manipulated in different ways. The flowers just illustrate that having a will is not a requirement to make use of it and all kinds of creative solutions could have arisen. Also, Meteion was a (quite literally) PET PROJECT of Hermes, and he was so careless at it that he even gave her a task that had a possibility of not being fulfilled, something Emet-Selch pointed out mere moments after hearing about it. Imagine what a full team devoted to design something better with the added motivation of preserving their own existances could have achieved.