I don't think changing the usage of system calls in the kernel as a reaction to Meltdown is a good workaround to make. This could lead to CPU makers relying on the workaround rather than fixing the issue in their CPU architecture. The other side of the medal is that this increases the motivation to switch to a newer CPU, which generates more waste, and the fact that CPUs running slower need to run longer, and use more energy, both of which is bad for the environment, and costs money.