I have managed to spend 600 hours in EU4 while owning no more than one third of this game's expansions and DLCs, and I bought most of them on very frequent sales, so I am not very afraid about the next one's value and replayability achievable even without spending fortune.
The differences between civ and paradox games DLC models are, yes paradox games release a *** ton of DLCs, but a) They have extremely long lifecycles, of at least 5 and up to like 10 years of support b) Like I mentioned they very often go on all sorts of sales and c) A huge % of them (most of them?) are quite modular regional content packs only adding content to very particular countries, regions, religions, cultures, government types etc so you feel free to not buy them if you aren't interested in a given segment of the game. Especially as d) Paradox games always get a *** ton of new exciting content via the free patches.
This is true. I have about 7,000 hours in EUIV, playing since day 1 almost 12 years ago - I had played some EU3 before. The over 10 years of support and new DLCs made the game immensely replayable and popular over that span - it still is. For an already deep strategy game, each DLC - well most - provided a ton of updates, enhancements, and new features that significantly altered gameplay, strategy, and tactics, keeping the game fresh for fans. A good example is playing as the Byz and having to find new ways to overcome the immense disadvantage they have against the Ottos. Other major enhancements came in the later stages such as mission trees, including branching historic paths for some major nations such as England going colonization route(i.e., traditional) versus an Angevin path with its own separate mission tree. Another example is the Teutonic Order, with a more traditional Prussian path vs a Horde path with a more Eastern focus.
My issue with the DLC is that, imo, the retail price point was exorbitant and restrictive - some upwards of $20. In addition, the cosmetic dlcs (unit skins, cultural flavor, map stuff) were not simply bundled with the DLC expansions, costing an additional $5 or so. (Actually, that is a major peeve of mine with DLCs in general and where real milking persists) Retail, EUIV is upwards of $400.00 to grab everything, but the sales have been pretty good, and I've paid a fraction of that as well as ignored most of the skin dlcs.
EUV looks like it will be successful, but I will probably avoid getting sucked into it for some years, especially as EUIV is still plenty enough for me. I have so much yet to achieve with it.
(Note: Paradox started offering a monthly subscription service for EUIV in which you get the whole kaboodle to play with. It's a great and cost-effective way to start playing it, if you haven't yet, as you can stop and start any time)