I think it is silly to think you would get game content (ie. any DLC released for the game after you bought the base game) fully free of charge afterwards, just because you decided not to buy it.
Is it really silly? Think about it.
When the game is new
a) preorder/buy the game because it looks interesting (50€)
a1) I really liked it, I buy DLC (+30€)
a2) I liked it, but not enough to buy DLC
a3) I didn't like it, and leave it be
b) Looks interesting, so I buy on sale (25€). b1, b2 and b3 like before.
c) I don't buy because I expect DLC and want to make one purchase with all included
When the game becomes old and the gold edition comes out
d) I buy the gold edition (20€) as I didn't have the game before
e) I buy the gold edition despite having the base game (20€)
f) I buy the DLC I was missing to custom build my own gold edition by hand (about 10€? probably more)
I think this are about all the different approaches you can take towards buying the game. A lot of people have complained here about DLC because they wanted to take the c) approach, until they could do d). Those are missed sales, until the eventual release of the gold edition and that's if the potential buyers even remember the game at that point. Gamers tend to have short attention spans, after all.
But let's focus on older games. I understand publishers/devs wanting to make money and hoping for e) and f), but let's be real: just how many people will actually spend money to get DLCs for a game years after having been into it, when you didn't want to while you were really into it? I suppose some a2) people might have increased their income and can afford DLCs they couldn't before, but other than that...I can't see it happening.
Now thing of what I suggested: upgrade people with the base game to gold edition for free. You still get all the money from a1) and b1) people. However, the people on c) may also buy the game at launch, because they know they are in fact buying the complete package ahead of time. And an a) or even b) is not only more money than d), but it's also money that comes early while the profit still counts.
I feel I'm not making myself clear, so I'll leave it at this: selling more early on is probably better for a studio than late sales, and I doubt DLC sells well later on anyway, so I honestly think the free gold update is a good idea. It's of course up to the publisher, not GOG, but I would like to hear GOG will be pushing publishers towards this as much as they reasonably can. It allows them to get DLC sale money without compromising on getting full packages.