

Mostly people are nostalgic for specific games because they played them in a special time in their lifes. Those early teens where you start to appreciate more complex stories, where you have the time and motivation to spend hundreds of hours on a game over a summer, maybe with your friends sitting near you all experiencing the game together for the first time. And as bitter as it sounds: There might be better games around now, but no game will be able to give you that experience.
I’m not really familiar with Stellaris, but with EUIV you really don’t need all DLC. Basically the DLCs are fleshing out one faction. So you get some special features and events for the Ottoman Empire, Venice or China that enhances how they play. If you do not want to play as Ottoman Empire, buying the DLC is pointless. If you have several hundreds of hours played, buying a DLC gives you something new and an excuse to start the Venice playthrough. I assume that they are handling Stellaris similar