Despite the fact I think I have an answer, let me say this is a good and important question.
In the status quo, Reddit, the mods, and the subreddit users have all coevolved with each other.
https://www.britannica.com/science/coevolution
They may not be entirely happy with each other, but they've developed with each other slowly over time, developed callouses, and generally worked out how to live with each other. Even if a mod here leaves and a mod there joins, the incremental changes to the community can be absorbed.
A complete replacement of the mod structure of a subreddit, especially a lot of subreddits at a time, will cause more chaos than you may initially expect. In ecosystem terms, imagine simply yanking out an entire species and trying to replace it by engineering fiat. Yes, eventually the ecosystem will resettle into some new equilibrium... but there is no guarantee that that equilibrium will be similarly healthy, biodiverse, etc. There's no guarantee it will even be an ecosystem at all; the Sahara Desert is basically an ecosystem that was so disrupted that it just straight-up collapsed. It is theoretically possible for it to end up healthier, but, well, look out in the real world at what ecosystem modification tends to produce and I think you'll be on the right track.
Replacing all the co-evolved mods with brand new ones is going to be intrinsically more disruptive than you may think. Moreover, the selection mechanism for the new mods is also likely to be highly disruptive; however they do it they're not going to be taking some mythical random sample, there's going to be some sort of systematic correlation between them all that will further tend to result in the communities experiencing rapid and non-trivial change. Plus, new mods will have no habits, no tools developed, no support structure from other existing mods, etc.
Continuing the biological metaphor, this is all coming at a time when Reddit is inflaming the entire community. Speaking for myself, I'm not particularly passionate about the API issue, I've just been following the community lead on it for the subreddit I'm a mod in, but I am definitely taking note of the loud notes of contempt coming out of Reddit in this drama. If this is the level of contempt they have for us all in vetted public statements, how much more contempt we must all be held in inside the organization when they can speak freely with each other. Consciously and otherwise, people are picking up on this, and things that a user might otherwise have pushed through will become reasons to leave for inflamed users.
The sum total of all of this is that while Reddit has the technical ability to remove and add mod status as they see fit, the community dynamics are more complicated than someone simply modeling the community as "the given set of people" may understand. They are living organisms of their own, if not ecosystems of their own, driven by a complex mishmash of relationships between the participants, and the level of disruption will be greater than you expect. Moreover, if you are not looking at the situation properly, the disruption will initially be smaller than you expect. The first day the invasive species is introduced into the ecosystem, nothing appears to be wrong. The first week after Reddit just boots all the mods and installs their own may seem like nothing much has gone wrong. The initial sound and fury will die down. But the sound and the fury isn't the damage; that's just the inflammation. The damage will only be seen in the weeks and months afterwards.
One of the most challenging things about managing communities is that for the most part, in my considered opinion, mass exodus is not the beginning of the trouble. It is the end. If you judge the quality of your community management simply by the membership numbers, you won't be getting any alarms until well after it is too late. By raw numbers I doubt in 2-4 weeks that Reddit will appear to be in any trouble. But the processes set into motion by such a drastic action will play out nevertheless.
Among the many disruptions a community may experience, rolling back to your original question, is a surprising shift in the community consensus, one that has not necessarily co-evolved with all the rest of the membership. A comfortable egalitarian community may become very doctrinaire about certain political matters, or, equally shocking to an established community, vice versa. It isn't just a question of whether a community is pushing a particular agenda, but which one. A sudden shift can be disruptive to the entire community ecosystem, even if there's a community right over there with exactly the same agenda happily functioning, because they co-evolved with it.