There's still one left (I just got one). The pricing model of the Retina MBPs is just craziness to me. To get larger storage sizes you have to jump entire model lines to get it, you can't just upgrade storage on them.
https://www.google.com/shopping/product/12203990123520892001
http://www.everymac.com/systems/apple/macbook_pro/macbook-pr...
You can still swap a drive in newer models, just with a PCIe card instead of an mSATA drive.
Simply put: Newer generation of hardware = newer storage interface.
Here is a replacement guide for a 2014 model Retina MacBook Pro:
https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Disp...
However, I've been pleased so far. The keyboard takes a little getting used to (because it is in the exact center of the laptop), but is good for typing overall. Good feel, no flex. The display is good too, with a wide viewing angle.