Why would it only be interesting at that point? At 500wh/kg, if the cost is low enough, you're already going to eliminate fossil fuels from basically everything except long range flights and shipping.
> Half the weight and double the range would be great
You don't need to quadruple energy density to achieve that. If you just half the weight (without increasing the total amount of energy in the battery), you're going to significantly increase range. The less weight you have, the less energy you need to move the vehicle.
> Plus, electrified ultralight "aircraft" start to have numerous advantages over the traditional 2 stoke engines.
I think you'll have plenty of benefits with ~300wh/kg (that's the target for many useful eVTOL aircraft).
The key challenge is you should redesign the whole aircraft around electric flight to get the full benefits. Look at NASAs Maxwell X-57 for an example of how that could look.
With 500wh/kg you can start taking over most regional flights. Yes, the range won't be as good as jet planes. But jet planes have FAR more range than they need because they don't design a special purpose aircraft for shorter range. They just put less fuel in.
But it'll probably take 10-20 years regardless of when we get good batteries because it'll take a long time to design and certify the aircrafts.