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

[CrownySuccubus]

As curious a phrase as it is, I've never taken that quote to imply he couldn't change course - he did after all in 5.3 (really, as far back as 5.0, although this became clearer with post-5.0 interviews.)

He didn't "change" course so much as he "detoured" it. I interpret Emet-Selch's actions to work the same as the concept of "inception" from the film of the same name. Basically, it's something that gets planted inside you, and no matter what you do or how you try to reason, the incepted idea always has influence over you. Emet-Selch could TRY to change...but only so far and so much.

Besides that, Emet was dead as of 5.3.

Moreover, Yoshi P makes zero reference to tempering in discussing his motives, e.g. here, or in any other interview of which I am aware. Ishikawa attributes his downfall to his kindness. It's a red herring to me.

Possibly, but I don't put much stock into that, because the writers have 1) admitted before that they tend to just make stuff up as they go and then sometimes change their minds later and 2) have outright forgotten or contradicted things they've said before. I'm partial to believe that they fully intended for Zodiark and Hydaelyn's chosen (the Ascian and the WOL) to be Tempered. I think they also probably intended for that tempering to be as different from normal primals as gravity is next to a black hole versus next to the Earth. Over time, the idea probably just evolved into something else and got dropped completely.

That's my take, anyway.