"Already" is doing a lot of heavy lifting, by the time the game comes out it will have been 9 years since Civ 6. That's 3 years longer than the gap between 5 and 6 which is the next longest gap in the series. At this point they're vastly limited in the things they can do without a complete overhaul on base mechanics we've been playing with for nearly a decade.
Would you rather they completely overhaul the current game and the original mechanics be lost to time? Of course not, the obvious next step is to make a new game.
Steam is great! But so is competition, and GOG has been one of the best of them. A healthy mindset and a strong focus on their relationship to the customer puts pressure on steam to do more than the bare minimum to stay in the lead. We're lucky in the fact that they're both mostly good companies to their consumers.
They each have their own issues, and I do question the effectiveness of GOG's strategy here, but GOG doing well is also good for you as a steam user!
Fair points for both, we're on the same page for the first point. GOG should be doing exactly that to mitigate the issue, and hope they have been but haven't been as successful with that technique. I'll give them the benefit of the doubt for it.
For the second, I agree that the majority of the issue are the storage space themselves, the others are tangential concerns. To me, a company that struggles to limit their file size has a poor take on how they implement features, it's a red flag that there are likely much bigger performance issues with the code. One doesn't mean the other has to exist of course, but they show up together fairly often.
I'm personally tired of game companies just throwing shit at the wall and not caring about the performance. They (well AA and bigger companies mainly) seem to have completely lost interest in doing anything other than the bare minimum. Does it work on the absolute latest hardware? Must be good to ship.
Are you forgetting the files are also taking up space on the consumers drives?
Edit: additionally, larger save file size typically(but definitely not always) means longer loading times. There are tangible consumer benefits to reducing save file size.
I'd more argue that the game company should be finding ways to reduce their save file size. 1GB seems ludicrous, though I don't know the system enough to know the technical reasons behind that. This is still a strange business decision for GOG as they don't have the market share to move the needle, the games affected will just sell less on their store until the game company doesn't even bother with it.