Upgrading to the latest and greatest Macbook is absurdly cheap since you can sell your old model for close to the original price
Oh how I wish that were true. I have a 2012 15" Macbook Retina (8gb/256gb) that I bought for $2200. The typical price on eBay is around $1200. So, I can get about half, which is your lament about the XPS 13.