Ecky :
I understand wanting to stay out of technicality arguments, but I'd argue the i3 is easily worth the $10 more. However, you could also go for a $50 motherboard with the Athlon and set aside some more cash for a better GPU.
It's a minority of games, but tell the people who play (just as an example) Guild Wars 2 that the GPU is the only important part. Go into any major town in GW2, and you're going to get 15-20fps with the Athlon, regardless of how much GPU power you have, and how much you lower the graphical settings whereas the i3 will still hold a playable 30fps.
Ask me how I know.
If OP plays any games that are severely CPU limited (basically all MMOs), the only upgrade for the Athlon is to replace the entire platform and buy a new Windows license.
GW2 is a very specific example. Furthermore, you specifically pointed to
large cities of GW2, which only accounts to less than 5% of the game and 0% of all PC games. That kind of example is not conclusive of the 860k's overall performance. I don't think the OP mentioned specifically GW2, or MMOs at all for that matter. You chose the worst category to misrepresent the CPU. Luckily, I happen to play GW2
and own a Athlon 760k (the previous Athlon generation) + r9 280. With the 760k OC'ed, I would get 1080p/60 fps with absolutely maxed settings just about everywhere except in big towns, where I get 30 fps. Keep in mind that's the max setting.
While I disagree with your argument and your choice of example,
I agree with you that i3-6100 is an overall better CPU than the 860k. That point was never contested by anyone to begin with. However, that "betterness" is rarely observable in real world performance. Personally, I would advise against grabbing the cheapest mobo you can find in a store. Computer is something to stick around for years, buy a quality part. With that said, there is nothing wrong with the i3-6100, just get a better mobo with it.