Okay, let's work with models.
Price of a media product = Value of individual product at time of release
That's an optimal model. Of course, things do not work that way - never have, and never will.
The obvious reason for that is that publishers need to make a profit, which might be in excess of the value customers might see in the product but still within the range of what a customer is willing to pay. Hence:
Price of a media product = Value of individual product at time of release + Profit margin
This works under the assumption that a product includes all of its advertised components. However, with on-disc or day-one DLC, that is not the case. What is advertised, or at least implied by the customer/consumer is: "The game includes the results of ALL WORK DONE before gold master", i.e. "You pay for everything the developer has created on this project to date, plus a reasonable profit margin".
If you have day-one DLC, the customer has reason to doubt that this is the case. In fact, resources that, regardless of internal structure, belonged to the project itself in the eye of the customer, are suddenly no longer part of it. From a marketing perspective, the simple act of announcing day-one or on-disk DLC lowers the value of the original product because instead of creating new value, it takes some of its "expected value" and builds itself around it.
At this point, it does not matter if the DLC feature was part of the original feature list or not - the customer will feel that the value of their product has been decreased. Suddenly, the value has decreased, but the price remains the same.
Ultimately, what is my opinion? On-Disk DLC is sheer idiocy. Telling your customer "we could provide more value to you at the same price, but we won't" is essentially marketing suicide. I don't want my local fast food joint to put tomatoes, salad, cheese and meat on a burger, only to then take the cheese off and sell it separately. But mostly, I disagree with it because this type of marketing strategy displays a sort of arrogance and bullying behaviour in game publishers that can only hurt the industry.