So a few things here:
1. Software support, AVX 512 is going to be way more fragmented then AVX is, how long before anyone cares?
2. Load store bandwdith, unless they double L/S (one of the most expensive parts of a core for both power and size) to be 4 times that of Zen then unless you can keep your work in registers 512bit avx is in the same place Zen is for 256bit.
3. What do you mean by "full speed". >Haswell FP pipes are all FMA and have longer latency then Zen so if the data is in registers even with back to back execution on Zen 256bits will execute quicker, Intel has 2 main pipes for FP that are 256bit, Zen has 4 pipes (effectively 2add, 2mul) that are 128bit. The execution time isn't an issue (speed), throughput in 256bit ops per clock will be a fair bit lower for Zen then >haswell because of load/store bandwidth, but are you going to buy an 8-10 intel chip anytime soon? I know im not dropping the $2500 a i7-6950 costs in OZ. The 8 core @ $1600 is far more reasonable
Its worth remembering TAM, how much TAM increase does intel get by adding large amounts of IPC, how much TAM does intel get by displacing exsisting FPGA/ASIC/DSP/etc by deploying new instructions/features into Core. What Intel does isn't necessarily the best thing for general computing, its the best thing for Intel to make money*. AMD has such a low Server TAM at the moment that all they need to do is general computing well to see a massive increase in TAM.
* I have no problem with this