They designed Argus without flying in mind because it was a simple way to give the illusion of a massive world we're visiting instead of a tiny island on a planet.
Visually, Argus is amazing in terms of looking like another planet. Flying would ruin that. To the point we don't have flight paths; we have instant teleports. They didn't specifically decide "okay, let's make a patch zone without flying", it was just a concession they made to make Argus as impressive as they felt they could.
They've already indicated not all patch content is going to have no flying; it will depend on the zone. And they said Pathfinder is a blueprint going forward, not that it will 100% always be in place for every bit of content, just that where flying will be enabled, that's the system they want to use.