For me, the problem is being asked to pay for what amount to simple balance and quality of life changes. Here's my take on the individual The Cossacks changes:
Major
Estates: Definitely major, a big change to the game and overall improvement. DLC could have used more focus on this, and it would have been justifiable.
Diplomacy: Also major, added a lot to the game (although may need to be rebalanced). Old system still in place for those without DLC.
Medium
Advanced Change Culture - Probably should be minor, not incredibly useful or important.
Build Directly to Army - Has no impact on how the game is played, just makes things easier on the player. Shouldn't be listed as expansion feature.
Improved Espionage - Espionage was almost universally thought of as terrible. Unfortunately, to make it worthwhile you have to pay money. This is a balance change and should have been free.
Horde Unity - A legitimate change that fits the expansion subject. Old system is still usable by people without the DLC.
New Subject Interactions - We already had one DLC (Common Sense) focusing on these interactions, but now they're split between two. That's frustrating. Also minor.
Native Policies - Basically the same system as without DLC, but with less micromanagement. Not an expansion "feature."
Tengri - As with Horde Unity, a legitimate change that fits the expansion subject. Old system is still usable by people without the DLC.
Minor
Name Your Heir - Not really significant, but it fits in the minor section
Victory Cards - I don't know that anyone asked for this, and it doesn't have much impact, if any.
Concede Colonial Area - Very significant change, would have put as medium. Although it doesn't really fit the theme, it is a good idea.
Distribute Spoils - A balance change fixing something that was broken. Hard to justify as an expansion "feature"
Construct in Subjects - Minor change
Threaten War - Very significant, could be medium change. Fits with diplomacy theme.
Forced Migration - Fits with theme, useful and original.
Declare Colonial War - Good idea, although does not fit with theme. Could be medium.
Raze - Fits with topic, also original idea for hordes. Could be medium.
So in my calculation, there were 2 major features, 6 medium changes, and 5 minor changes. There were also 4 balance changes that were sold as part of the DLC, and 1 multiplayer balance change.
I always liked PI's DLC system, as I felt like I was paying for significant mechanical improvements. But paying for balance changes and having to buy 3 different colonial DLC's is frustrating, and I wish you would reconsider how you do things. Even worse are things like AoW's removal of "don't give me provinces" toggle, forcing you to buy AoW to transfer these provinces away.