I think it was good to move to a 3 digit numbering system for the iGPUs, so it becomes easier to distinguish from the 4 digit CPU numbers. The old 4 digit iGPU numbers looked awfully similar to the CPU numbers for those not in the know, e.g. 4670 CPU and 4600 iGPU.
Not only this. AMD already did use a similar numbering system with HD 6000, HD 7000 etc. Without Intels numbering change Skylake would be HD 7000 as well.