I'm using established psychological and business concepts

I meant looking at one data point and extrapolating a pattern and causation. All your data point shows is that CK3 sold more than CK2 over a shorter time span, but maybe not as much more as one might expect. There are a lot of other valid ways one might interpret that data.

CK2's model includes the sales of CK3

Well sure, any sequel is impacted by the game it's succeeding. That's true for pretty much any product.

The question is, did CK2's model hurt CK3's sales long term? Paradox's DLC model is a long term strategy, so initial sales are a bit less interesting than longer term sales than in other games, but they're still indicative of interest and how successful future DLC releases are likely to be.

To get that answer, we need to compare Paradox's model vs similar games that use a different model. That's the same way you'd do it in stock market analysis if you're trying to identify why a stock movement happened.