It’s not hard to imagine, given what we see with the iPad 3 and 4, what an iPad Mini with a Retina screen would be like with today’s technology. Its battery life, portability, or performance would suffer significantly. (Probably all three.)
Apple didn’t make an arbitrary decision to withhold Retina on the Mini to save money, upsell more buyers to the iPad 4, or “force” the first generation of iPad Mini owners to upgrade next year. They chose not to ship a Retina iPad Mini because it would be significantly worse than the previous iPads in very important factors.
Could Apple have slapped a Retina display on the iPad Mini? Of course they could. The question is should they have? Heck no, since doing so would probably have made the iPad Mini a stout, sub-par tablet with abysmal battery life, and essentially placed it far below the standard and quality that people expect of an iPad.
For now, the iPad Mini is as good as it’s going to get without sacrificing the quality of the user experience.