Sidegrade maybe IF you talking about 32nm and up to the 6c/12t even those was faster on X79....but 22nm/Ivy-Bridge was faster and overall improved(less heat&power)also X79 has variety of Xeons up to the 12c/24t + as you said AVX it's a big deal when it comes to the gaming especially in past few years....
I think it comes down to what performance numbers are personally considered to be a "sidegrade" and what platform gains you are pleased of.
I was running a w3680 (this is a xeon i7 980x) @4600ghz with 2400 ddr3 cl10 as a daily (all voltages within architecture specs) and comparing none avx loads like CB 15 :
4.6ghz 6core DDR 2400 X58 : 1104 MT , 144 ST (i was not able to find a screen of my daily but the 4.8ghz oc shows some 4.6 numbers (the X5675 number) that was the same for my final oc).
4.5ghz 6 X79 DDR 2600 (disabled 2 cores of my 1680) : 1230 MT, 162 ST
So we talk about ~ 10 % increase of MT-Performance and 11 % ST Performance.
Yeah it is 100mhz less but 4.6 is not giving much more. 166 ST, so we talk about 13 % ST and some gains in MT. Even if you assume 1300 MT what will be an unrealistic gain for just 100 mhz more we talk about 15 %. I can just validate the 166 ST due screenshot by myself. (have not tested the 6core performance of my 1680 in MT with 4.6ghz).
I was able to pick up a xeon 1650v2 setup for cheap 1 or 2 years before i decided to go with the 1680v2 and after testing it for me personally the performance gains were to less to take the hassle of rebuilding the system.
So i would consider even Ivybridge 6core to beeing a sidegrade coming from a good 6core Westmere/Gulftown OC in none AVX loads.
Now i am happy with the 1680v2 since my use case has changed and i can utilize the 2 more cores, AVX and the higher memory bandwith. Especially the artificially cut down write performance of X58 was weired.
And in my opinion back in the days X79 did not offer some must have feature that was forcing you to upgrade from X58 to X79. PCIE 3.0 was nothing you really needed and it was still lacking the native USB 3.0 support.
Nowadays the PCIE 3.0 is nice, the NVME modding possibility is also nice (nvme with x58 is only possible with drives coming with own orom or usb efi bootloader) and the option to use high core count cpus
is even nicer ;D as you said. So X79 aged really better.