I think this is a very positive option, as the option to purchase the DLC permanently and individually is still available. Paradox confirmed they continue to look into making the same subscription system available to other titles. It remains entirely optional, with DLC purchases not going away.

and many more improvements to the core game experience.

The Crusader Kings II Ruler Designer, so you can create your own starting monarch.

The Game Converter, which lets you export your saved game to Europa Universalis IV.

10 portrait packs of clothing and faces for rulers around the world.

14 music packs, including two heavy metal scores.

12 unit packs, including new army sprites for your medieval forces.

13 major expansions, including the fierce Norse of The Old Gods, the eastern riches of Rajas of India, and the scheming vassals of Conclave.

If you wanted to buy all of it you're looking about £228 whereas the subscription is £3.99 / $4.99 / €4.99 per month. Like other Paradox Development Studios titles, it has a lot of DLC available and it can be a little overwhelming on both the cost and what to pick from. The subscription service is now live on Steam, giving access to everything that the game has to offer.

It being free also makes it a good entry point for Paradox strategy titles. Even though Crusader Kings III is out now, the classic Crusader Kings II is tried and tested with multiple thousands of people still playing it regularly.

Paradox Interactive are branching out with revenue models and they're now doing a subscription for Crusader Kings II, which itself is free to play.

Crusader kings 2 free ruler designer