Some of the migration aspects are coming, but somethings even NetApp can't change. I'd like to see "data move" in ONTap 8.x be able to move CIFS/NFS shares as transparently as it is supposed to be able to move LUN's, at least within the same controller. I guess that would essentially make such moves possible with entire aggregates as well, but the complexity and duration of such an operation would have prohibitive risks in today's world, I would think...
Upgrading disk sizes is a phenomena that has only recently gained speed. Since the availibility of consumer grade ATA/SATA disks that were forced to compete on a purely per GB price market, sizes have expanded rapidly. It hasn't really been a necessity. You could by 144GB disks for many years, for example. The pace was slower. Perhaps this wasn't one of the highest priorities when WAFL was made.
The file system would have to be modified to understand the modifications in physical storage that were occurring somehow. I think calling WAFL a mantra is pushing things a bit far. Given enough time, money, and human resources, it probably can be done, but NetApp is a company that needs to make a profit and not an academic institution developing solutions for historic corner cases. It may become necessary, but it hasn't been so far, I would think. I don't believe there are any competitors that offer this functionality either.