I feel pretty strongly on this one - I'd much rather all the mechanics be released together, as that allows the mechanics to meaningfully interact with each other. A large collection of smaller DLC makes it difficult to have interconnected mechanics which play off of each other. It also means there are more combinations of possible DLC to take into account for patches and mod requirements.
While that's generally true, there are approaches they could do to accommodate for interactivity. And then the conterpoint example would be the actual expansions with Civ 6 had a bunch of mechanics that didn't necessarily interact that much with each other.
Ways of allowing separate DLC to interact would be, imho:
1) Having DLC that sits on top of game systems that are in the base or that are added in a free patch. Spitballing an example would be say faith as base and Relgion as bonus DLC - you could do mechanics that interact more with the faith layer. Same with culture vs tourism, where some basic logic can give a great work if the tourism DLC is enabled and culture bonus if not. Or they add a Logisitics system that's just a morale penalty for distance from your capital in the base, but an actual mechanic with the DLC enabled. Alexander say gets no morale penalty in either.
2) Open ended systems you could add independently to. Policy cards are an example - new DLC, new policy cards for that game mechanic. Diplomatic options are theoretically an example (religion DLC enabled, accept my State religion is a trade/diplo option). The era score system is sort of an example but as it's always additive could get complicated with the scaling (Religion DLC is enabled - 10 more ways to get era score - but the cap is increased). They sort of did this with the Loyalty system where it was very stand alone and they later added in interaction with religion, happiness, grievances.
The above definitely still requires more planning and potential bugginess, I'd agree. It also lends to more generic currency boost sort of bonuses (which something like Humankind almost entirely does) while Civ seems to be going more towards the bespoke goal type of bonuses (using an apostle on a different terrain type in an ally's territory gives a great work!)