Author here; RAM is so much more expensive than disk though. Two 500G M.2 NVMes for a RootFS in RAID1 are basically max ~$150 which is I think much cheaper than a single 64G DDR5 ECC RAM module [I don't have exact numbers on me, but ECC RAM is pricey]. It's also a lot harder to debug when things go wrong because you lose the machines state if everything was ephemeral.
We run a thin base OS on the boxes and then VMs on top which we consider more ephemeral. The frequency of needing to update that base OS is v. low.
I think there's a case for building a custom PXE booted RAMdisk image to replace the install though; something like what Equinix Metal (formerly Packet) do with Tinkerbell (https://github.com/tinkerbell); they call it an OS Install Environment, but the idea is a small lightweight linux install agent that can DD a golden image onto the disk (vs. an install each time).
