You mean potential as in 1~8 potential? If yes, you know that potential is not stats, right? If a rider is dominating, f.e. Froome, it doesn't mean he should have 8 or 7 potential. He's already at the peak of his career, there's little reason to give him a potential that will allow him to improve a lot further. The same goes for Nibali, and pretty much all other riders who are at their peak.
The low-potentials system for these major riders of Daily is not random or because the stat makers haven't realised that Nibali and Froome are very good riders. It's thought to avoid having overpowered careers, that is, having dozens of riders with 80+ in their main stats after a couple of years in careers, and based on what has been tested so far works quite nicely