There still is a cost differential between 1TB RAM and 1TB disk, and that matters in the real world.
Varnish lets you decide where you want to spend your money, if you want to run straight out of RAM, it can do that, if you want to have a bigger cheaper and slightly slower cache on disk or ssd, you can do that too.
You have written enough publicly that we could probably cook up a GPT2 level simulacrum with your posts. Useful for code reviews, discussions, etc.
Aren't we saying the same thing? The RAM is there to support the disk and the disk holds the data (the asset), so your article is disk centric. If disk was fast enough, there would be no ram. The ram is just a tool to support a latency target.
Back then you couldn't buy machines with 1TB of RAM, most systems were 32bit and limited to about 3GB usable RAM.
Varnish was written with 64bit in mind from the start, but it also had to work on 32 bit systems, so we had to be able to use disk-backed VM to get past a couple of GB.