FFXIV is not an open world game. The instanced structure should probably have clued you in to this fact.

Even in open world games, however, zones still have a finite lifespan. Elden Ring was the best open world experience that I've had in a long time, and even there you'll find that backtracking to earlier zones feels largely trivial even if there's a missed achievement or boss that you're going after for completion purposes. That's a function of the game experience - when you've 'conquered' a zone and filled in the map with waypoints, it ceases to feel dangerous (even moreso when you're able to fly). Scaling up mobs does nothing to change this feeling. I'm not really sure why Warcraft was ever mentioned as something to aspire to here, because it was pretty much built on top of the carcasses of dead zones.