Yes that is what I intended to say. Rational people aren't willing to pay for a game that they already own.
Like I said the fanboys will buy it on release. But most people, knowing that they already have a complete and decently functioning WWII game in HOI3 to play, would either wait for a 75% off Steam sale to buy HOI4 or would not bother with it. Where is the incentive to buy HOI4 on release with the knowledge that it is going to be buggy and needing expansions to be played properly? (this isn't an implication of Paradox, it just seems to be the norm these days for all companies)
A WWI game would have a greater percentage of sales on release because there are fewer alternatives. Sure you will still have those people who don't buy anything on release. But the types of people that would think "why should I buy HOI4 when it will be half-playable and HOI3 is fully playable" would be more likely to buy a WWI game on release because Paradox doesn't have a WWI game. And like I said before: a WWI game would help keep Paradox relevant. Would you rather have Paradox be known as the company that keeps making the same games over again or the company that releases new and innovative games? A WWI game would be better for Paradox's reputation and would be more effective at attracting people who aren't regular Paradox customers and the people who don't impulsively purchase everything Paradox releases.
If HoI 4 has an extended timeline that goes to the begining of the century it'd make it niche. But the issue is that the eras do not fit and there is too much complexity on having two great scale wars since you can't really end the first one 'properly' so that the build up to the 2nd one appears at a more appropriate time.
Build up to WW1 (lots of wars around the world before that; Russia vs Japan, Russian revolution, Balkan Wars, France vs Morocco, Italy vs Turkey etc) as well as the build up to WW2 (political changes in many countries, arms race etc) is something that a game mostly centered around war is not going to portray really well. But if the devs manage something like this and call it HoI4 I'd buy it instantly.
The problem for a stand alone WW1 game is that the build up is of relatively minor historical importance as I don't think people expected that a massive war would take place, while the conflict was escalated mostly in Europe. Sure some expected a big war between some huge countries, but I don't think there was enough to say that this sort of thing would have happened. In WW2 it was pretty obvious.
