I would actually not have bought the bundle. That way you did not appriciate the content in it´s true context. Buying a DLC for CK2 is for me the yearly gaming- highlights, the full "unwrapping - spirit". In this "more is less" and would have ruined the spirit for me.
Why did you need them all at the same time? Unlike you want to start 3-4 paralell playthoughs, you will not benefit from all the 5 DLCs listed above in any single playthrough.
And in response to your pricing opinions: Games are expensive today. Buying a DLC or 2 each year that greatly expands a great game like CK is far less expensive than buying a new game with the same ratio. That would have been the alternative for me. An expensive one for sure and far less rewarding(for me). Looking at the pricetag for 10 of them at a time should be compared in this context.
Older DLCs should not become cheaper just because. The basegame could though, but they have been giving at away for free and might do that again
Game is old but very much improved. Even if game was released 2012, it´s a far more a new game than any other game released 2012 where no/a few improvements have been done.
Players still play World of Warcraft. it´s about 15 years old and cost yearly ~150€ just to play + any new expansion(another 30-50€ each). I don't blame them, I do activate my account at some times but playing CK2 is for more pricy
A couple points I wanted to respond to in this post . . . on the whole, you make great points. But for the prospective marketing/sales person, there are a couple of possible fallacies that deserve to be thought through fully . . .
Why did you need them all at the same time?
Without doing a LOT of homework to pin down exactly which features and functionalities were "in" each dish of the DLC smorgasbord, how was I "supposed to know" which ones I did and did not need? When I look at how the features, content and functionality are distributed across each dish, it seems quite clear to me that the distribution favors an "own them all to get your best value" quality. This might have just been an accident of timing in development of various features. Or it might have been a very much intentional strategy. I cannot fault them for adopting such a strategy, but at the same time, consumers are not so stupid as to fail to perceive the strategy and like I've said, it clearly hasn't won them any "most consumer friendly publisher" awards.
Your point that "game is old but very much improved," is another good point for consideration. Now that I own the whole thing and can see at a glimpse the content, features and functionalities and make some quick appraisal of how they enhance the game (not to mention all the improvements just from patches to the base game), yep I agree! Game IS old but also IS very much improved!
As far as I can tell, you and I might be the only two people on the whole Internet who have stopped to make this point!

Moreover, there have been LOTS of loudmouths out to make the exact opposite point: game is old and prices are too high and your 'nickel and dime us' strategy is repulsive!
Stop and consider a game that has a "rabid" fan base; I'm sure we can all think of one. I'll think of . . . uh, whats that Zombie game set in Kentucky? Holy Toledo will you look at that!
More Zombie games that you can shake a stick at! Sheeze . . . I own the damn thing and it isn't a "bad game," in fact it has quite a few neat features, and the poor developers have had a rough road to get where they are. But they, IMHO, also have some very poorly conceived product/design visions. Like most notably, NO actual NPCs in their game! But try to go in any online community where their fan base dwells and tell them about that, maybe suggest that it is a real deficiency that sets them back. You'll see what I'm talkin' about!
Point being: it isn't actually "THAT HARD" to become BELOVED by an at least outspokenly zealous base of users (or so it seems) and Paradox is comprised of clearly a LOT of delightful, lovable, virtuous, and meritorious people! And yet, "beloved" is the last thing which this "DLC Swarm" strategy seems to have evoked.