FF16 stealing even more dev time from FF14

[Seraphor]

not it doesn't

This is right. The content IS there, it's just distributed in a different way.

The content that in ShB made up the Bozja/Zadnor zones, the relic grind and the large scale duties, is instead spent on Island Sanctuary, Variant/Criterion dungeons, and the new Deep Dungeon.
The design is less efficient, less impactful, but from an outside PoV it seems to have taken more or less the same resources. So it's not about dev time, its about design choices.