I don't agree. DL would have its original character and charm -- and worldwide customer base -- while DCA would pull in new visitors, who would also try DL while there. So DL would still be busy, while the trendy, new crowd would be going to DCA.
Overall, though, I think the right answer was to build Star Wars land only at Hollywood Studios in WDW, where they actually had the land and the capacity. As it stands, it seems that they are trying to cram WDW (with its "blessing of size") into a city block in Anaheim -- and along the way destroying, or at least overrunning, the character of Disneyland, simply because it does not have the room.