So if you take the price of the i3, the performance and overclocking of the 2600k, the ECC support of the e3 xeon, and also somehow get a free motherboard out of the deal, you can have an intel cpu that beats the 8120 on all counts?
That is amazing!
Now back to reality, you can't pick and choose your features and prices. You need to pick a single CPU and that is it.
Pick the Xeon. Cool, you got a CPU with ECC just like the FX-8120. Probably a bit faster at base speeds. But it costs a lot more, and can't overclock. A loser.
Okay, instead pick the 2600k. Now you have some higher base clocks, and you can overclock further. Except now it costs even more, and you lose ECC support. A loser.
Okay no problem, instead take a cheap i3-2100. Now you have a cheaper CPU. Except after you pay for a motherboard, it's not cheaper anymore. Also, it performs worse, can't overclock, and lacks ECC. A loser.
Nobody has ever said you can't get a faster CPU than an 8120 by paying more for a high end Intel. You can, it's true. But you give something up. You pay more, you lose your ECC support. Or if you go crazy and build a Xeon system, you pay more, lose your unlocked multiplier.
Intel makes a lot of great CPU, but they lose in this specific case.