Previous civ games, and almost? all 4X games in fact (and most strategy games in general?) have been sandboxes. You freely choose the civ, the map script, and then off you go. The game consists of a world with civs thrown there and broad static rules (not really changing with eras), according to which civs follow snowballing accumulating growth to victory. There is no broader structure beyond what occurs as a result of AI or human civs individually do.
Thanks for your writeup, it was an interesting read!
Old World and Sid Meier's Alpha Centauri are two 4x that come to mind that are not strictly what you define here as sandboxes. There are narratives within the game and choices that guide your empire. Especially Old World.
Weirdly, though, my experience with Civ7 has felt more like a sandbox than Civ6. Civ6 for me was like doing my taxes: when I had it figured out, I had it figured out. Getting there was a slog. There was little to derail my playthrus, and I had to decide early on which route I was taking. It forced the player to create the rails, but was no less railroady in the end. The end-game of Civ6 in my brain is like a scrambled acrylic painting of bright colors and pre-determined decisions. It is the same reason I dislike the Pandemic board game: there is, very often, the best decision in Civ6, objectively. If you want X, you should do Y... and that's it. The curves and colors of the game betrayed this idea of a sandbox. I never really felt my empire was growing and living, but felt instead that it was a collection of color-coded tokens on a board.
In contrast, Civ7 very much opens up that idea of freedom in a Civ game (to me). The legacy paths are often misunderstood as requirements. People talk about the treasure fleets and how specific that is, without remembering what a science victory was in previous Civ entries (go through set steps to achieve victory). But legacy paths are just that: paths. And for me, the branching pathways are better than 1 pathway. I can pivot based on the circumstances the game throws at me. Sometimes this will be a sudden war, but other times narrative events, impactful natural disasters, or an invasion from an independent power. Things feel more alive in Civ7. I almost feel like I could walk away and the world will keep on without me. That's a magical feeling.
You'll never please all fans of a huge franchise though! That's a pretty impossible task. I'm glad the devs went bold here.