The main problem that I see with Stellaris is that it shouldn't be compared by the same criteria as the other games as Stellaris, as it stands right now, is not a true grand strategy game, it's a 4X game with grand strat elements, better to compare it to SoaSE or AoE2.
The other main weakness of Stellaris is that it is the first of its kind among PDX. CK2 is an old game with a lot of patches and DLC under its belt, HoI4 while not having the advantage of years upon years of post-release improvements has three other games in its series giving the developers plenty of points to draw from. EU4 has both of these advantages, while Stellaris has neither.
Buying a Paradox game on launch is like buying a plot of land and hiring a company to build your dream house on. You start with a nice plot of land, plenty of space, some nice trees and there is a little creek down in the backyard, this is the base game. The price you paid the company upfront covers the cost of the construction of your house, they lay a good foundation, build the frame, drywall and roof it, this is the free updates. But you still have to go buy all the furniture for the house so you can actually live in it, a fridge, bed, couch, TV, among other such things, this is the DLC. You don't have to buy furniture, but if you don't you might as well be living in a shack.
I moved into my CK2 and EU4 houses after they were sufficiently built up to live in and bought all the furniture upfront and was able to live comfortably in them. My Victoria 2 house was fully finished and turn key. My HoI4 house just had the final studs put up and the toilet installed. They were still digging out a space for the foundation when I bought my Stellaris house. The point being that while my HoI4 driveway may not be done yet, and I might be taking a dump in the rain because my Stellaris bathroom doesn't have a roof yet, I am sure that they will one day be comparable to the beautiful mansions the company is known for. And while the crystal chandelier of my CK2 castle isn't installed yet, and my champagne waterfall in my EU4 palace may not yet be working, I know they are only going to get better with time.
TL;DR Paradox sells lifetime passes to amusement parks that are still under construction.
