Why are alisaie and alphinaud in Dawntrail?

[Striker44]

Because SE is terrified of making this game without the Scions.
Scions have a lot of popularity, and they do not know how to pull the plug on those characters and let others take the spotlight. It's a 'dug up your own grave' situation, because the entire narrative since SHB keeps elevating the Scions to be the most proficient people in certain matters IN THE WHOLE WORLD.
When it comes to inter-dimensional travel, we shouldn't allow other characters to grow in that spotlight, we should call G'raha and Y'shtola immediately. It is dumb not to.

It could be any character, not just Wuk Lamat there, it would build resentment to not having the Scions around, or letting them do what they know best. Because, again, they are the most competent people in the whole world as the narrative kept dumbing everyone around and making the Scions have all the answers to literally everything. And the Devs don't know how to get away from it in a healthy manner to make their appearances rare and yet exciting.

The popularity of the Scions is definitely one key factor. I wouldn't say it's a matter of "not knowing how to pull the plug" but recognizing that "pulling the plug" would live up to the reference and kill the game. It's one of those ways that people whose only experience of the FFXIV community is these forums would benefit from widening their view, because we complain a lot about the Scions here, but they're immensely popular among the playerbase at large.

For the other part of "allowing other characters to grow," I see it as a natural matter of human comfort. We've worked alongside the Scions for years now and we all trust each other. If one of them is knowledgeable on a topic, it makes sense to call in the person we both know has the knowledge and who we already trust implicitly compared to some relatively random person we don't really know yet and haven't built that trust with. Think about it in real life - if you need some work done, and already have a long-time friend in that line of work who you know is good at their job, are you going to start randomly Googling other people you don't know to get the job done, or are you going to just call your friend?

The hilariously out of touch writers instead bench the characters that SHOULD have been in the frontline.

I wouldn't call them "hilariously out of touch"; I'd say the opposite - they're very much in touch with the reality of the Scions' popularity and the effective need to include them in the stories they write. I see the "out of touch" take as thinking you could just effectively dispose of the Scions and it wouldn't drop the bottom out of the game.