Senseless fanboyism like this annoys me as much as senseless hatred and is unecessary.
The fact that it (province improvement) will be a core mechanic of the game, and that it can easily affect balance, is enough of a reason to include it in the free patch. I don't think anyone in this thread has suggested that the DLC should be free, no, but rather that mechanics like this belong to the free patch. When 1.8 came around, Paradox did not force us to purchase DLCs to get the new provinces and this is a similar situation.
It is different to having the religion-specific mechanics be in the DLC, or even the parliament, free cities, subject interaction, etc. Base Tax, Manpower and now Production are all important values of how this game's core mechanics work, and leaving something like their improvement outside of the free patch is pretty likely to cause future problems for patching, since it will limit options unless Paradox completely ignores who for some reason did not purchase the DLC. Furthermore, it can limit modders, given that to make use of this mechanic in a mod, the DLC would be necessary, even if that is (IMO) a lesser problem which was far worse back then when we would get expansions.
Frankly, ideally most players will get the DLC, but such an important feature should be in the free patch, given the current DLC model. If you think that everything should be in the DLC like you have just stated, that's fine, but it'd require a radical shift from the current DLC model back to the old expansions one and likely would only be viable for EUV+. And it would, of course, kill MP.