And that's a problem, the way I see it.
The main advantage of the World Axis is that it's an open cosmology. There's no end to the number of astral dominions there may be out there. They are probably not infinite in number, but there are definitely more than the Great Wheel's 17 outer planes – and there are many of the Great Wheel's planes that don't belong in the World Axis. The dominions need not have anything to do with alignment, but can either embody other concepts or just be plain weird. You need a world without shrimp? There's probably one out there.
The Great Wheel, on the other hand, is a closed cosmology. It doesn't allow for any outer planes other than the 17. Sure, you can have a demiplane in the Ethereal, or an alternate Prime, but not a fully-fledged plane. The Great Wheel is the result of box-checking, and when there are no more boxes to check there's no more room for expansion.