When I bought my Macbook Pro a year ago, I was feeling so internally guilty about spending $US1500 ($2,011) on a laptop that I only paid for 128GB of storage, a decision I've regretted about three times per day ever since. This solution isn't cheap, but at least it will stop me whining.
The solid state drives in Retina MacBook Pros and MacBook Airs aren't actually inaccessible — they're just difficult to reach and a non-standard part, meaning they can't easily be changed. OWC has a new product that should help with that: two SSDs designed to fit in 2013-or-newer MacBook Pros and Airs, with 480GB and 1TB of space depending on the drive you buy.
The kits ship with all the tools and instructions needed to do the change at home, but the price is a little scary: $US400 ($536) for the 480GB, and $US650 ($871) for 1TB. Admittedly, those also ship with enclosures for you to hook up the old drive from your MacBook, but whichever way you cut it, it ain't cheap. Still better than buying a new $US2,000 ($2,682) MacBook though.