There are no good JRPG writers left in the industry in general, and having a manga writer around to "consult" isn't going to help. Hell, having a professional writer consulting does nothing even outside of JRPGs - the new-gen Tomb Raider games had a novelist working on their scripts and their stories were still trash.
The problem with XV wasn't the writing, it was the completely slapdash management and the fact that S-E worked on the game for so long that large chunks of the team didn't even know what made it into the final game and what didn't. It wasn't a badly written story, it was a badly told story.
It was both a badly written story and a badly told story.
It was largely incomprehensible, especially if you didn't watch Kingslaive or the anime shorts.
It was a mess. The whole game was. But a competent story with better writing could have distracted from the game's other major problems.
The game director and studio management all have their part to play in the game resulting in what it was, but FFXV's story isn't the only FF game with a shitty story in isolation. It's been an ongoing trend since FFX.
You can make excuses and blame it on the studio mamagment and the dev problems that continue to persist game after game over at Square Enix, but the fact is, the people working on the games creatively managing the projects, are failing.
They need better management, yes. But they also need far better writers.
And your point about JRPG writing is nonsense. I disagree entirely. But that's also rather subjective, so each to their own really.