My prediction: In 5 years 16GB will be fine.
Why? Because RAM requirements passed from "We need more, it'll speed things up" to "We've got enough" a while back, and there just isn't much you can do with the extra. Once you're no longer space constrained it just ceases to move the needle.
I'm not defending Apple here, but generally it's hard to think of something that would require 32GB of ram that doesn't better belong on a server. Once you get to that kind of size you'll probably want ECC or stable uptime or other features you'll get on an actual server rather than a machine you turn on and off several times a day.