The question with subscriptions is at what point does it become cost-effective. That would be very dependent on the nature and age of the game.
Stellaris has a few DLCs, but is heading in the same direction as EU4 and CK2.
HOI4 took a slight difference in making it possible to buy the first few DLCs up front in purchase, but is now in the normal purchase for new DLCs.
Imperator, so far, has been providing free DLCs.
EU4 and CK2 both have a large number of DLCs, some of which relate to the game play and others provide a visual/audio wraparound. CK2 is now at the end of its development and CK3 is coming.
For any of them you have to compare the costs of acquisition vs the length of time you will use it. I have just looked at the costs for EU4 with all of the DLCs. As purchases, that works out £200, with none of the content packs. Of those i have spent £56 and there are another £90 that I have not taken. But I have spread that cost over the last 6 years, and some were bought discounted. If I was a new user wanting to play the game, and guessing that it was likely to be replaced in the next 2-3 years, would I be prepared to pay £200 to £350 up front, or would I prefer a subscription which could give me all of them immediately.
At which point does a subscription become more expensive than acquisition? My guess, if EU5 appears in 3 years time, then a subscription should be, for that length of time, lower than acquisition. From a Paradox view, it is better to get a lower income rather than no income. That could suggest a subscription rate of about £40 to £70 per year, on a basis that Paradox will recover the purchase costs after 5 years, but by which time they will be selling the new game and DLCs.
For new games there is a different view. Paradox would prefer to sell the DLCs and get the money immediately, or have a higher subscription rate. But a customer would want the subscription rate to be lower, so that it would be more cost effective over time. If we assume that games are replaced every 8-10 years, then a subscription rate, from the start, should be higher, in total, than the cost of purchase, but as the game is developed and more DLCs are added then that rate should increase since there will be a shorter period within which it will be valid.