As for GOG, they need to stop bait & switching out original content then shrugging if off as unimportant, as it both kills off any incentive for collectors to "double dip" here, and also drives those who do want the original content back to abandonware sites that still host it (not to steal it but merely to retain the version they paid for but no longer have, even on 'you buy it, you own it'
This should just never happen, and borders on criminal when it does.
When you buy a game, and even if in reality that is just a license, that is the version you paid for.
That said, I am now going to play Devil's Advocate.
It is quite common, in online music stores for instance, to sell you a track or album, which you need to download as soon as possible, because there is no guarantee your download will remain available past a certain time frame, often specified. I personally don't agree with that policy, but as I always download immediately, I've never been impacted.
Now GOG haven't gone down that road, though you could argue, that in the current instance and similar, they are indeed doing that, though you still have access to some form of the game you bought, just not the original. In fact, any game that receives an update where the original files are no longer available is just that.
I am also pretty sure, that in the fine print with every purchase you make at GOG, they cover themselves for such changes to game files we have bought.
It is possible to look at this issue both positively as well as negatively.
Though for my money, I would rather retain access to original files, and even earlier versions provided by GOG before I bought a game.
Taking a look at what Bethesda have done for an instant, and I don't know if GOG are bound by certain conditions when a game is made available to them, that also means the provider retains full control over what files are provided at any one time to customers.
I have read where those extra mission packs are considered detrimental to the original game, such that they are not a good way to promote it, and despite collector's wanting every single related thing, Bethesda for instance, might consider the game is better off without them, that maybe they should never have been available in the first place, that they lessen the game experience overall.
It is also possible those extra mission packs were wrongly provided in the first place, without the appropriate permissions, though if they are still included at Steam that at very least begs a question on the viability of that notion.
---------------------------------------------- However, nothing excuses the run-around MarkoH01 got. That is inexcusable, and both GOG and Bethesda should be ashamed.