I have been with Paradox since the original Europe Universalis and Hearts of Iron. Those were horrible times that I do not want to return to: frustratingly waiting 3 to 5 years for a sequel to add new features to the game. Dedicated superficial patches stopped after 6 or 12 months. Any additional patches was developed/tested on the programmers' (Johan's) personal time.
Now, games like Crusader Kings 2 and Europa Universalis 4 are staying alive and comprehensively updated for years. This is what I wanted 15 years ago. Pay up and do not stop!
I agree that this proves the best way to improve on a game we love, while giving PDX a net-income instead of spending a huge amount on developing a "new" title every other year, so we would have CK5 and EU8. However there should be a time to stop and gather yourself and ask, would this be the time to make some major upgrades in the form of a brand new game, building on what we already learned (know it can´t be a direct extension, but still have some similiarities since I suppose most features are well worked out and you support them in the game).
Real "buckling" will result in declining DLC sales and gameplay hours. If those trends are recorded as true on Steam, Paradox will then cease DLC development and begin working on a sequel. The choice is yours!
Personally I went away from CK2, it just didn´t seem to "fit" anymore and don´t think any DLC will have the ability to change it up enough to do so. Will probably still buy the DLC to try out and support PDX (since I want you to create great games). But I believe when "buckling under it´s own weight", refers to performance issues, not sales issues.
As a responsible developer you should know when to draw the line, not just because of sales figures. Also since you don´t want to lose the customer base that would have bought CK3, just because you kept milking CK2 DLC until sales were noticeable enough.
There must also be some voices and ideas in the office that floats around, that can´t be done under the current engine... Make the best game ever possible, be it DLC or a new game. But don´t base it on the sales (makes it sound greedy and is not reflective to how great the game is, just how many fanboys you have that support you, myself included which only have roughly 20 hrs into Conclave), base it instead on how the game is handling, how the game you want to have can be made in the current engine...
In short, don´t wait making a new game until customers have been scared off buying DLC.