Actually, I would consider a retina screen a reason NOT to buy an iMac! Applications that are not designed for retina would suddenly have small UI elements. Oh sure, it would be great for apple-written programs but they are not the only company making software.
That's not how it works. OS X's implementation of HiDPI for Retina screens means that there's a difference between the "addressable" resolution for things like drawing windows and the actual resolution. By default, the difference is a factor of 2 in each axis, meaning that a Retina screen in HiDPI typically has the same screen real estate as a typical screen with 1/4th of the total pixels.
Applications that don't have inherent support for HiDPI mode will simply be scaled up to the best of OS X's ability. Unless they did something abnormal with the text display in the app, it should adapt automatically to the HiDPI fonts. The images will simply be pixel-doubled, so that they'd look (approximately) the same as they did on the lower-resolution screen, only using 4 pixels to paint each pixel in the image.
Anyway, the most logical direction for a Retina iMac would be a 4K screen at 3840x2160. If the default scaling factor was still 2x, this would mean that a 27" Retina iMac out of the box would actually have less screen real estate (and larger UI elements) than a 27" normal iMac out of the box, though undoubtably there would be a setting to switch to 1.5x scaling to get the same 2560x1440 of real estate available today.