HPC is not the real use case (target). The percentage of HPC compared to the rest of the target market (public cloud) is insignificantly small.

The real question is do you trust any potential co-tenants in a public cloud that share a hypervisor with you, when said hypervisor does NOT have Spectre/Meltdown fixes applied? (There is only one correct answer...)

Also you can fairly easily mitigate this in a public cloud setting by running your VM on a dedicated hypervisor. AWS calls these "EC2 Dedicated Instances". Interestingly enough for most compute sizes the cost different is negligable. I imagine that isn't the case everywhere / for-everything, but if you A) need pre-Spectre/Meltdown perf and B) need protection against co-tenancy attacks, paying a few more dollars for a dedicated hypervisor host seems like a no-brainer to me.