only that they are farther and take longer time to transit. 

Looking at the map, you'll see one route by way of the south of Java, thru Lombok Strait, Makassar Strait, Celebes Sea, Philippine Sea to Japan.  The second alternative route goes by Timor, Banda Sea, Molucca Strait, Philippine Sea, then to Japan.  Both routes bypass the Strait of Malacca, Singapore, and South China Sea.  Large Japanese oil tankers and US carrier groups have taken these two routes frequently, although not constantly.

Chinese Navy has long noticed these two alternative routes and are taking measures to get familiar with these two farther routes in case future needs arise.  Chinese Navy had quietly sailed thru these two routes in exercises not too long ago. 

On surface, the three artificial islands China is currently constructing will control the Strait of Malacca and South China Sea.  But there is more than just that.  The need to constructing 3 large airfields on 3 islands is also for the control of the air over these two alternative sea routes!  If you draw a circle of 1,500 km radius, you'll see what I am talking about.  For this reason, the 3 airfields on the 3 islands are a must, not just an option.  They will be done.