If the problem was systemic, you'd hear about it on these forums with other sanders with more frequency than the Kapex issues.
This particular problem pops up once in a while. It's a good ol' regular here.
Personally, I've only used Festool tools for the last 6 years, but never once an issue with the plug it cord I keep at my MFT workstation.
From what I've read here some people have problems all the time, others never. I think what Bob mentions above is right on the money, the number of connects and disconnects play heavily into it. And the way you use the tools.
For instance, I am sure you and I have very different ways we work with our tools, you work on your MFT, and I've been runing up a ladder all week sanding the edge of a roof with my DTS, ETS125 and Rotex.

Cord and hose taped to my ladder to get 5,50 meter high, and having to change between sanders at height, multiple times. This was a bit of heck week for me, because the condition of the paint was worse than originally anticipated and I decided to use my own ladder for what seemed to be an easy job, instead of renting scaffolding. To my dismay I had to strip it all to bare wood instead of just some light sanding.
I have worked like this before, with other tools before I got into Festool 11 years ago, and I never had problems with cords. But the Plug-It breaks all the time, one way or the other.
Given that you've had multiple issues with different tools, perhaps the unknown origins of the tools you're refurbing has something to do with it? And we know an electrical issue can spread like an STD when swapping cords and tools. So one bad connection can result in many over time if you're not careful. I'm not discounting your observations, obviously your tools failed you. But I don't think this a widespread product or design failure on Festool's part which is what you're suggesting. For my own part, the plug it connection is fantastic. And one failure in 13 years is perfectly acceptable. It is a wearable component.
Mostly answered above. It is not the origin of my refurbished tools, it is the number of disconnects and the way of working. I'm out in the field all the time with many different tools. With a total of 17 Festools I think I have to buy 2 or 3 new cords each year. I also have a number of other tools from DeWalt, Elu and Metabo, I never have to replace a cord.
The idea of the Plug-It cord is very nice, it's just not strong enough for rough circumstances out in the field. If you work in the shop all the time, sure, you're going to be fine. But I think it could and should be done better.