It was a flop because it was intended for a process node that Intel was delayed on for years. It had massive problems to the point of not really making sense when backported to older nodes.
I don't think it's accurate to say AVX-512 was backported. The original Skylake consumer CPUs released as the second generation of products on Intel's 14nm already had space reserved in the CPU core floorplan for the AVX-512 register file. That space didn't get used until the Skylake server CPUs shipped, still on 14nm several years later. AVX-512 support didn't arrive in the consumer desktop product line until Rocket Lake, which was backported to 14nm but was not remotely the beginning of the AVX-512 story.