I don't think that being able to upgrade really matters. In fact, even if you can upgrade, you will soon run into barriers.
I've upgraded my 2007 MacBook Pro to a 500 GB SSD and 6 GB RAM. The CPU and GPU or everything else can't be upgraded.
So where is a Retina MacBook Pro worse with respect to upgradeability? The SSD can also be swapped - and it's probably much easier than swapping the SSD on a 2007 MacBook Pro, which has the disk deep inside. Well, the RAM cannot be upgraded on the new model... but wait, I can't go beyond 6 GB on the old one, either (actually, it's already above the official 4GB limit). So if I order a Retina MacBook Pro with the maximum RAM, it does not make a difference at the end of the day.