Do you mean also e.g. Mac (and Linux) versions of the games, and the various language versions like Italian, Spanish, Russian etc.? Is there some specific reason you'd want to keep all versions and languages? I'm just interested in keeping the English Windows versions at the moment. I might also download English Linux versions for any games that have them, but at the moment I don't consider it that important.

As long as GOG servers are healthy and up, i'd think having only one set of local copies is enough. The GOG servers are still there for downloading anything that had gone missing for some reason. Of course if you are afraid that hard drive goes completely kaputt in an instant, yeah then you'd have to redownload it all all over again.

But at least HDDs usually tend to start giving signs if they are becoming to the end of their life (with SSDs it might be different), and I tend to move my backed up files to newer hard drives anyway whenever I buy some new bigger hard drive(s) as they've become much more cheaper, longer before the old hard drive(s) would die. It is not like I keep e.g. my personal files from 15 years ago still on some 15 year old hard drive (or floppy disk or whatever), they have naturally been copied to newer media over and over again over years. That's the beauty of (DRM-free) digital files, easily moving them to other media.

My GOG game installers and extras are currently about 2 terabytes (1315 titles, only the English Windows versions). It feels a lot right now, but I presume they feel like peanuts in the future if and when I have e.g. a 500 terabyte hard drive and I've moved my whole GOG collection there.

1: I dont save "saves" I replay from the beginning; if i stopped playing there is a good chance i forgot it all anyways
2: I use an internal HDD designed for backup and daily use
3: I keep them as they come from GOG with all extras downloaded.
4: i have an external array and use a "tower of hanoi" system for backups

It's unlikely, but I'd like to plan for the possibility that I turn into a macfag some day. Having both Linux and Windows versions is useful for me, as there are cases where one runs better or worse (or not at all) than the other. I once had a 1.5TB mirrored raid implode. It wasn't fun, though I managed to recover at least most of my stuff in the end.
My library could be close to two terabytes then. My connection is pretty fast and I don't really play games anyway.

yes
sometimes
external hd's and or blu rays
i keep them as installer files or the steam back up files
the blu rays are the permanent back ups

And? 2 TB is nothing. I work with film, photography, audio ... keeping and archiving those data volumes are a problem :). 2 TB isn't, really. Why do you buy them? I do understand there are collectors out there, but as a method of collection I'd have thought physical objects not digital copies would be much better.

Yep, all my games from gog, hb, indie royale, etc... fit on a 4TB HDD with room to spare, it's really not a big deal.

A digital collection is easier to keep in mint condition and it's great to have something for later when you got more spare time, even if it's decades away. Most of us will be gaming a lot when/if we get even older, I suppose.

I've always exhaulted the ability to archive your games on GoG but don't really do it.

So I'm now having a play with gogrepo (great code), to download my library for safe keeping (and quick install later)

I always backup my backups o.O

these days 2tb is nothing even 4 tb is getting small
my ps4 has a 500 gig hd and the damn thing is already nearly full

even singe side blu rays are getting annoyingly small with just 25 gig