I think you're surprised because you still view VI as a finite installment in a series rather than a new attempt to create a legacy style game.
The industry has changed since V and long-term support games have tended to be the bread and butter of major developers. Square Enix likely wouldn't be alive if not for the continuous revenue provided by XIV. Blizzard is dominating the competitive industry with overwatch. Even smash bros has switched to a long term, seasonal release model.
Firaxis was faced with a complicated situation for VI because so much of it retrods V's design territory,
That was by intent - Civ has always tried to follow Sid's mantra of trying to keep the complexity of the game comparable to earlier instalments, removing features from older games and adding new ones. Civ VI is already bloated because it's departed somewhat from that, but the basic game engine has no room to expand further. The sorts of games you're talking about are things that have more freedom to expand, or can add more meaningful content in small doses. New civs are always welcome, but functionally they aren't much in the way of new content and don't do anything to change the game experience.
Most non-Paradox strategy games still follow a traditional model, and the Paradox games are just longer-lived versions of the typical marketing strategy. Consider Total War games, or the Endless series which have somewhat regular patch, DLC and expansion content, last three to five years, and are then supplanted by a new release - indeed these days Total War games other than Warhammer have very little post-release support: Britannia had none and I doubt Three Kingdoms is getting much more since Warhammer III is due this year. I'm not aware of a 'legacy' system for any major strategy game publisher.
and early adoption was pretty middling. It took two years for VI's subscription to match V in a divided playerbase.
Civ VI was a relative commercial failure at release - that's not a case of a "divided" playerbase, it's a case of a broad consensus that the game wasn't very good. That's been fairly typical of Civ games in general - they're perceived to start out worse than their predecessor and in time come to equal or supplant it. We don't have Steam figures for Civ IV (at least that count the entire playerbase) but I'd expect Civ V had a similar trajectory relative to that game. If Civ VI were to reach a point where there was a more universal feeling it was better (or at least more accessible) than Civ V, people would switch. Civ IV diehards evidently didn't harm Civ V's popularity.
The civ model has diminishing returns because it's just rereleasing the same content over and over again,
That's what a franchise is, when you boil it down. There's no sign that any 'diminishing returns' are setting in with Civ or with any other franchise based on a similar model - we still have unending streams of Assassin's Creed games with cosmetic makeovers, and Creative Assembly has only increased the rate at which it releases new Total War games. Civ V was likely the most successful Civ has ever been - Civ VI hasn't done as well as far as we can tell from the available metrics, but that's a single game rather than a trend and one often held to be inferior to its predecessor despite having more features. Even if Firaxis were concerned it might presage a broader trend, you don't create a legacy game from an already-finished product - if this experiment works with Civ VI possibly they'll actually take that approach with Civ VII, but as I recall you've been stridently arguing that the legacy model doesn't work anyway because there's less buy-in each time.
The best way to break the model, of course, is to create a game with can be continually patched and expanded and built out over a longer period of time, delay the inevitable reset button as much as possible.
Which is not possible with Civ VI as built, and Firaxis has given little sign that they either have an interest in frequent patching or that they're planning on using patches as a regular balance pass or a way to make substantial changes to gameplay. The latter is a necessary part of a legacy pass system to keep the games fresh.
You're taking a highly stereotyped view of game marketing - one size has never fit all. The legacy system is visible because it's a good model for the most popular genres of games - things like Destiny or Diablo for which minor content changes can promote continued play to obtain the latest loot, and which have a competitive multiplayer element that encourages people to access the new content ahead of the competition. The content is also, as I understand it, often time-limited, so that you have to play during that season to obtain those items before they're gone (at least I think Diablo 3 did it that way).
Strategy games by their nature are built on core systems that can't change that much over time, and minor content additions like a new civ don't have the appeal or ability to spur an extended period of new gameplay that a new gun or gameplay area in Destiny might.
The fact that people keep approaching VI/VII as if the franchise is exactly the same as it always has been is baffling to me. The industry has evolved and online support is not only becoming a standard for most games, but practically required if a game wants to be successful. We have been seeing the evidence for years as Firaxis has been interacting with the community, putting out patches every few months.
At a very much lower rate than the sorts of games you're describing, and patch changes have tended to be relatively minor for the most part. The timing of major Civ VI patches hasn't been any different from that of Civ V's. Firaxis tried departing from the expansion model with Civ V - the message they received loud and clear was that people did not want a DLC system instead of traditional expansions. DLC has become normalised since, but once again with Civ VI Firaxis tried a couple of early civ DLCs and abandoned that model once the main expansions came out, which suggests that they didn't find them to be a success this time either. There's good reason for treating Civ VI the same as Civ V: its development history so far has been exactly the same. That may or may not change with Civ VII, but it defies logic and empirical evidence to suggest that Civ VI has not been handled in the same way as Civ V, or to imagine that one experiment once the game's core development is over implies that Firaxis will take a game designed in a traditional framework and try and force it to become something else.
