Do Libreboot supported hardware devices, which appear quite old, still receive CPU patches? If not, it is probably time to stop using it. Coreboot may give you a fighting chance however.
Then again, newer CPUs have a host of other problems, like only supporting platforms where rolling your own boot firmware / UEFI is next to impossible.
Personally I think the CPU has a much lower attack surface than something like UEFI (microcode updates are also pretty tiny), so I would rather have an outdated CPU than an outdated / vulnerable UEFI implementation.