If Civ 5 remaster comes out, I can imagine people complaining about anti-snowballing in it, such as science penalty for settling more cities, slow workers, roads costing money, etc. given how many people started their civ journey with Civ 6.
Civ 4 may be the best game for some, but it's not the game that made Civ popular as it is now. Its mechanics and systems are more old school and dated. I don't think its return on investment would be great for the company.
I might have something to say about your Civ 5 point in time, but your Civ 4 point does dovetail with something I've been mulling over for a while.
The designers and financiers of any game want to get the maximum number of players possible to play it. The designers because it's gratifying to see people liking your work; the financiers because of $. For an ongoing franchise like Civ, one thing that will mean will be a constant pressure to reach out to new possible markets. So consoles, Switch, etc. And new marketing models: DLCs, microtransactions.
People who liked the franchise up to 4 think of it as a computer strategy game, and think that every iteration after that has involved dumbing the game down. There is one poster here who is quite vocal in this view, and quite colorful in his expressions of it.
But I think your core point is correct:
that fanbase is "old school" and, most importantly for my purposes here,
small relative to the customer base that one might hope to draw by building one's game to appeal to console, Switch players, etc.
I think this is an especially powerful tension in the design of a Civ game, in part because of that starting idea that some older fans have about what a Civ game should be. For that fan, Civ
shouldn't be for everyone; in fact, it should be for a pretty small cadre of the kind of audience that originally played computer games: geeky, socially-maladjusted intellectual loners. (Sid's crack about how, if they had friends, they wouldn't be playing his games). So the more that Firaxis "dumbs the game down" in a chase after a wider playerbase, the more disappointed the old-school fans become.
But on the other hand, one can only go so far in making Civ appeal broadly before one gets away from the core of the game itself. It's a pretty rare gamer who has the patience to carefully micro-manage multiple systems over a 300-turn strategy game to a final victory. Civ is never going to get the casual Candy Crush player.
Where all this has been leading me is to wonder, not really about Civ, but about some potential new, indie company. One that wants to make an old-school, Civ 4-style game,
and is content, financially, with the niche of players who would buy such a game. My questions are "how big is that niche?" and therefore "what could such a company stand to earn if it made such a game?" and then "would the budget for such a game be sufficient to fund the design of a Civ 4-style game (that would be received by that niche as a fun successor to 4)?" In effect, my question is, "would it be possible, in this day and age, to fund the development of a Civ 4.2?"