Thats ideally how it should work. And sometimes it does to some extent. Rome total war got DLC some eight years after its release and some new features for everybody with it. DLC can elongate the life of a game significantly which is a huge plus in my book. (Even though I wouldn't put total war as a positive exemple for other reasons.)
However, DLC is often an excuse to ship a game as barebones / low effort as possible to make additional money with DLC later. Paradox is a prime exemple for that I think. Crusader Kings 2 is bearly enjoyable without some DLCs. And alot of the DLC is ridiculus expensive. Nobody can tell me that the price of close to 1000€ for CK2 with all its DLC is justifiable. You pay 20€ here then 15€ there and before you know you have payed well over a 100€ for a game to enjoy which should have cost 50€.
If the game is solid as is, and it is worth its money, additional DLC is always great of course. Either it us worth its money, then buy it, or it isn't, then enjoy the solid game without it. But expecting people to pay for stuff that should be in the game to begin with is rediculus.