Sun did something similar many years ago, but they abandoned it in later UltraSPARC CPUs. Was it that the threads were starved? Did they find that less cores but faster was better? I can't find out much detail about it.
Perhaps HN user bcantrill will give us the inside scoop :-)
I used them in the heyday. There were workloads where they were fantastic, but anything that needed good single-core performance suffered. It also often required much more tuning of parameters to perform, or recompilation, etc. For me, it also coincided with lots more need to use SSL, which had been optimized well for x86 but not Sparc. So now you were dealing with more complexity via SSL offload cards, or reverse proxies. Basically, just too fussy to make them work well outside a few niche areas.
Maybe not a big factor, but it also was bothersome for sysadmins because much of the work we had to do was serial, single core, etc. Meaning it showed it's worst side to the group that usually signed the vendor checks.
Perhaps HN user bcantrill will give us the inside scoop :-)