When Eu1 sells X copies, Eu2 sells a small fraction of that and then Eu3 is back up to X copies, it is a kind of indication that a large majority of paradox gamers want an open ended sandbox game.
Interesting line of thinking.
But wasn't, to the casual observer and non-fan, EU2 almost like a paid upgrade. This isn't meant as criticism, I'm not bitter at all about EU2 or it's pricepoint, but I remember various magazine reviews over here writing something along the lines of "Same as EU1 with some minor upgrades, should have been a expansion." And EU1 and EU2 indeed look VERY similar.
I figure a LOT of the folks that bought EU1 and didn't like it enough to become dedicated fans skipped EU2 because of that similarity to EU1.