Stopped reading there. No one asks for "maintenance", what is asked is to not remove what was already there.

The ddraw library that came before this GOG version mentions Microsoft Visual C++ Redistributable 2010 package as requirements, and that has Windows XP and Vista as requirements, so in theory the GOG version should run on XP and Vista just fine.

You get original version for your ancient hardware. This is the only version working with BattleNet too. So you get both original (which works on older and newer systems) and enhanced version.
Other than that you got answer - they put worst computer they've tested it on as minimum requirement. They probably don't have computers as ancient as XP ones.

Actually, it's not really the original version, it is still somehow patched and won't run at all on Windows 98 if you don't have at least DirectX 9.

The installer itself doesn't run at all on Windows 98, and even by copying the installed folder from my Windows 7 machine to a Windows 98 VM, i've not been able to run it properly — managed to get to the menu once, but it crashed when I tried to connect to Battle.net, and now I get an error about ddraw.dll again.

Since I could not perform a clean install, I did have to "cheat" my way around to even get it to start, and wouldn't recommend anyone running Windows 98 or any other unsupported OS to use it to connect to the Internet anyway, but still, it's a bit sad that the original version, which can run on Windows 9x/ME etc. isn't actually included, for the retro-gamers who'd like to play it on their old hardware.

Yes the exe file is different size too among other things, wish there was a changelog of what fixes to the game was done.

Still it's not a replacement for actual CD / disc image of the original game, if you want it playable on a retro system. GOG do include disc images in some of their games though, but it's not currently included for Diablo.

Then you'll never understand, because to keep old code working you have to maintain it (or update it) to run on modern architecture, even if that means emulation. So what you are asking for IS maintenance.

And you are asking for them to support years and years of old code and methodology, whilst you can't even be bothered to read a few paragraphs. That just says it all.

Which isn't at all what I'm asking, but I'm the one that doesn't understand, yeah.

Modern architecture applies to both software and hardware. If you want to run it on an old machine then just get the original CDs. GOG don't create installer packages for DOS, Windows 95 and Windows 98 as these are UNSUPPORTED operating systems. Even Windows XP is an unsupported operating system.

GOG has never been about making OLD games run on OLD systems, but instead OLD games running on NEW systems.

How stubborn can you be? I'm talking about the original files being modified to no longer work on the machines they used to work on, not about having GOG giving us services for old OSes, I'm not asking for that stupid galaxy nor have I been complaining about the installers either. You can still play the DOS games of GOG on DOS, they just happen to be bundled with DOSbox. That's the point and your argument is invalid on top of being completely off mark.

Firstly, Diablo 1 is not a DOS game, it came out on Windows 95. The original requirements were Windows 95 and Windows NT. Games from Windows 95 often require community fixes and patches, or DirectX wrappers, in order to be able to run on modern Windows OSes, which is what GOG distribute games for, as I already stated. So what you are asking for is for GOG to provide you with the original unmodified files for an OS that is long since unsupported by everyone. That's not going to happen, it isn't GOG's business model and they shouldn't have to support long dead OSes.

Wow, so you start throwing the insults. *clap clap*. Well done.

Did you try only copying the Diablo.exe, DiabloUi.dll, DIABDAT.MPQ, DiabloUI.dll, SmackW32.dll, Battle.SNP, Standard.snp, and Storm.dll?

What the hell is wrong with you?
First, read the OP, then try to look up "stubborn" in a dictionary.

Questioning about 'system requirements', we must separate SR for GOG releases and SR for normal original game distributives.

Concerning the old games, GOG's goal now is not 'reviving the old classics', it's just 'making money selling bad-wrapped and semi-fixed distributives of good old games'.

So I personally see no more reasons for me to continue supporting GOG with my money. I'm an old-school gamer. Old-school gaming has no more anything common with modern GOG, alas.

Nothing's wrong with me, I'm happy with the release, it's what to expect from GOG. You, however, are the one with the angry language ("How stubborn can you be?", "What the hell is wrong with you?").

As for the word stubborn, calling someone stubborn is more often than not intended as an insult. Also, it has been made clear to you the facts of the matter as to why GOG do not support running the game on old hardware and operating systems (nobody can be expected to support Windows 95 any longer) and why the game files are not identical to the original ones (as the original files will not work on modern Windows OSes without applying community fixes and patches - which is why GOG releases use tools like DDraw wrappers, 3DFX wrappers, DOSBox, etc.) which therefore impact upon the system requirements, yet you are the one who won't accept this reasoning (hmm, that definition...).

stubborn: "having or showing dogged determination not to change one's attitude or position on something, especially in spite of good arguments or reasons to do so"

Except wrappers don't modify the original files. Also I'm pretty sure old Diablo runs as is on "modern"(give me a break) OSes. Not sure what you're trying to prove, but it's definitely out of blind corporatism, not virtue of truth.
And yes you are stubborn, because you keep saying even now that I'm asking for things I didn't even. This is fallacious.
All I'm asking for is for GOG to sell a game without breaking it.