Free-to-play games need paid cosmetics stay afloat, and live service multiplayer games need dedicated servers that cost astronomical amount of money to maintain, so in those cases paid cosmetics are the necessary reality.
Paid cosmetics in singleplayer games, especially in games that can have mods available to essentially get them for free, however, does not look like a healthy practice at all. In fact, I actually saw other games restricting cosmetic-related modding to remove competition for their paid cosmetic dlcs, so Firaxis suppressing modding scene in this aspect is not at all implausible scenario.
"Cosmetic" dlc is actually necessary in some regard for single player games if you are using income steams as your argument. Essentially, programming/game engine development takes a lot longer than art. Cosmetics dlc allow an income stream justification for the art department, allowing them to stay employed as full time resources, rather than temporary resources that get hired near the end of development and then let go after release, as used to be the practice.
Having said that, for Civ, leaders (and civs) are essentially already 'cosmetic' DLC for that purpose - something like a leader is probably 95% art department, 5% programming. We saw that in the New Frontier Pass for Civ 6 - something to keep the art department busy/employed while the programmers were working on the Civ 7 engine.
Same dealy with modding - both Civ 5 and Civ 6 had enormous amount of modded Civs competing with the paid DLC Civs, and Firaxis did zero to suppress those. If anything frankly, Firaxis seems to be pushing modding in the direction of mainly cosmetic, leaders, civs, etc. with the lack of source code.
So I don't honestly see this as a big issue, because to me Leader/Civ DLC is basically a step away from cosmetic DLC. But by that same argument, purely cosmetic DLC seems like a bit of a waste of time for Firaxis as they could probably get a lot more bang for their buck adding that extra 5% of dev time to make a leader/civ. So I don't know if we'll see much of these beyond launch.