Prices on GOG range from five to ten bucks. SW on 3DRealms is ten bucks, probably not a whole lot more or less than you would pay to GOG if they sold it. 3DR also provides "extras" for SW that you can get without having to buy it first. GOG makes you purchase the game before you can access the "extras", which really aren't that much of an incentive to buy the game, when compared to the game itself. If it's about the "can download it anytime if you buy it on GOG" thing, then all you have to do is save the installation executable in a backup folder on a backup drive. And the "Extended Download Service" from RegNow (the people who handle purchases for legacy games at 3DRealms) is just a fancy way of saying "We'll keep a download link handy any time you want it for the next two months". You could save yourself three bucks by just keeping a copy of your receipt for the transaction, and email RegNow with those details if you need to re-download the installation executable.
Contrary to popular misconception, "demanding" things left and right and all the rest of that "squeaky wheel" crap is more likely to cause others to ignore you. Or tell you what a little bastard you're being.