Correct me if I am wrong, but would not that make the folks who bought it one by one really really angry?
Mixing all the paid patches minor DLCs together at a less greedy price would be nice, however.
I'd see absolutely no problem whatsoever if they do it based on time.
Like having the DLC become free after two years, or perhaps slashing the price by half every year (until it reaches <$5, then make it free), or something of the sort.
Anyone who bought it early would've enjoyed it at launch and for a long time by then, probably to the point of exhaustion, and is much more concerned about the newest features for the current DLC.
Not to mention, DLC features being incorporated into the base game means devs now have the liberty of working more deeply with them and truly integrating them to core game mechanics, like what's happening with estates now (and is precisely the reason why it hadn't happened before).
This come from someone who buys them at launch without issue, but who had a very hard time trying to recommend this game to friends exactly because of the DLC list.
All they see when they go to the steam page is an endless list of DLC with an egregious tag at the bottom, it can't possibly get any unfriendlier.
Having old DLCs become free would have the opposite effect, they would look at that list and think "hey, look at all this free stuff I'm getting" (even if the base game were to be more expensive), and then bam, PDX got a new customer on their market to buy their newest DLCs.
A subscription service isn't really the answer I think, as it gives the feeling that you're signing up for a long term commitment, even if that's not the case, and might scare some people off.
In the end, I'd really like to see the sales chart for these DLCs through time, just how much money would PDX be losing if they made 2 year old content free?
It is a known fact for games that the vast majority of a game's overall sales are done within the release week, how much of that is true to PDX DLCs?
How much is Rule Britannia and previous DLCs making them
right now?
Is that value worth gating off so many potential customers?
I don't have this data, so I can't answer for sure, but those are questions that should be asked.