I just made the same switch you are about to make. I ended up going for a late 2013 i7 27". It benchmarks almost as fast as the 2015 i7 on geekbench. Yet I only paid $1600 for it shipped on eBay vs $3000 for a similar 2015 model. The way I see it:
1) I wear glasses, so the retina isn't really that different to my eyes when I'm sitting at my desk.
2) similar benchmarks mean a similar time to obsolescence. I just paid half price for a computer that will get old from a processing point at the same time as current computers - maybe even later since my graphics card has less pixels to push.
3) very few people are building web pages, videos, etc that can fill the 5k retina. By the time they do, the benefits will justify the cost.
Of course to each their own, if I could afford it, I would go for the 5k. Point is at this point in my life, I have better uses for a grand and a half.
Your point may have some validity, but you'd need to point out a few things along the way.
1. Wearing glasses or not wearing glasses does not make or break a Retina screen. There are some people who see the dot pitch difference and some that don't. Personally I cannot glance at the two screens and tell them apart. But after 3-4 hours of working, the non retina screen makes my eyes sore and I get a headache. The Retina screen doesn't seem to do that for me, so I do prefer that.
2. I took a look at Geekbench along with quite a few other offerings, and they really were not close. Where the issue comes into play is when you start to throw in graphic performance along with the score. You are driving more pixles on the screen so playing games will take a hit. But that aside, it looked like your getting closer performance between a 2013 i7 and the i5 vs the 2014 or 2015 i7 units.
I will give you that the cpu performance on most computers over the past 3 years have been pretty stagnant and the gains are marginal at best. I personally would doubt that someone with a 2013 iMac would say it ran any slower that the newer RiMac units. Perhaps there are certain apps that it would show on, like multi thread video or audio encoding, but how much and how often do you really do that each day??
3. Web pages or video for me is not the reason to go for a retina screen. It's for me that the screen dot pitch is so much tighter it causes less stress on my eyes and I find it some instances the characters do look better as they are smoother.
Now the bigger point you failed to mention in your post when you compared the price. You bought your 2013 iMac off ebay and paid a great price of $1600. That is almost 1/2 the price of the retina iMac that you say you were looking at. Yet, if your purchased iMac had a problem with it in the first year, you are paying for your own repaires as it's unfortunately out of warranty. You also don't get the option of buying the AppleCare to get those 3 years of service. So yes you did save some money but also took on the risk factor at the same time. To some people that would be fine, but to others that is not something they want to take on.
