If you shift each group 2% towards the Pubs
A two per cent swing is not unusual for a presidential election.
I agree, however it seems unlikely to me that the current white vote numbers are the new ceiling for Democrats. I'm more inclined to think that their numbers will go back up at least 1% - 2%, based on historical shares since the 70s. Mondale lost the white vote big, but everything eventually rebounded. So with that in mind, if they got their 2% swing but with the Democrats normal average share of the white vote, they mostly end up right back where they were in 2012.
So if 2012 was just a really good election for Republicans in terms of their share of the white vote, then that is not good for them going into 2016. As for non-white voters, I can see that swing.