Minimizing risk is a wrong way
The picture you shown tell:
- Variant one -> Patched
- Variant three -> Not compatible with AMD processor architecture.
- Variant two -> near to zero risk.
Care: near to 0 / 0 give an infinite more risk than zero.
If you are ok with near than zero risk, stop reading this! ;-)
Spectre vs Meltdown
Both exploit (or security failure on processor architecture) are based on different feature.
From a lot of time, constructors are creating processor with many cores, and a lot of features for making them work quicker...
From spectre.pdf paper:
More broadly, there are trade-offs between security and performance.
Not same feature... Reading user space memory from side-channel is simplier than reading user space memory by making cpu bug by exploiting speculative execution.
Then, if understanding Meltdown is easier than understanding Spectre, also mitigation against Meltdown is easier then mitigation against Spectre.