The CPUs are vulnerable because of the exact way in which they are being applied to a problem. Speculative execution is not inherently unsafe. Whatever future predicted memory prefetching shenanigans are going on in my CPU over here have absolutely ZERO impact on your CPU over there. Certainly someone could figure out a protocol/system/architecture that capitalizes on this notion that "2 different CPUs are indeed different CPUs".
One can see how any perspective here still causes trouble for Amazon, Microsoft, et. al., but that was a business risk they signed up for the moment they intended to squeeze every last drop of subscriber revenue out of the hardware. Why should everyone else on earth have to suffer crappier performance by default because of the business/software practices of a select few?