Latency/speed shouldn't be ignored, but most people aren't going to notice it, with the possible exception of an APU user.
Gamers and to a certain extent AMD CPU users will notice it. DDR5 comes as high as CL40 or 44. A 32Gb kit that is say, 6000 mhz CL30 isn't very expensive, but 64? That will cost money. If it's cheap, yeah why not but it isn't.
I don't see Windows filling 32 Gb with cache data or anything else. Do you have something specific in mind? Haven't seen it use more than 16 though i haven't tried very hard to use up as much RAM as possible.
I agree that 16 is too low for a new PC unless the budget is hella tight, but my whole point is that double that is more or less the only option as i don't think 24 Gb is a sensible choice.
32 isn't going to be filled up in a hurry but it's what a new system should have. In a few years, memory usage approaching 24-32 may become more common but we're not there yet.
64 is double
that. And it comes with a latency penalty unless you're willing to fork over more than $200. If it's needed, there is no question. But if 64 is not going to be used or even stressed for 4-5 years, then spend the money elsewhere and enjoy a fast 32 Gb kit.
Some people do need 128 or more, but those are very rare cases. The vast majority is only now moving to 32.
If the price is right, or if it's something you really want by all means. Hell, i do have 64 Gb but i don't care about latency and run it stock. I also haven't used 16 Gb of the 64 so i know i didn't need to buy that kit. *shrug*
I don't want to make anyone's mind up about this, i just want to discourage people from believing that having 64 Gb is neccessary or useful "future-proofing" because it is neither. What they choose to do is up to them.