Apparently Apple believes that Thunderbolt 2 changes that equation.
It looks to me like they've tried hard to make sure that the included GPUs are adequate for 90% of their users, at least for today. As for tomorrow their philosophy has always preferred replacing the machine rather than upgrading it piece by piece.
Yeah, that seems to be their solution. Actually, it seems like their solution is to pick a GPU that is adequate for 99% of their users at launch time. Which is not ideal, since everyone who buys a Mac Pro is going to be paying for super-premium graphics, even if they are buying the machine for the Xeons.
It looks to me like they've tried hard to make sure that the included GPUs are adequate for 90% of their users, at least for today. As for tomorrow their philosophy has always preferred replacing the machine rather than upgrading it piece by piece.