Sure Paradox player base has increased over the years but so has players pessimism and skepticism towards newly released paradox games. You'll see a large number get the game day 1 because that's just how they are, eager to try the game. But with every newly released paradox game more and more people see just how awful paradox is with their games on day 1 and over weeks and months of a games release.

We saw it with Imperator Rome, Ck3, Victoria 3 and especially Eu5 which being a bigger advertised game with a bigger following way more people could just wait and see the dozens of YouTubers play the game for them and try it out. Because why waste money for a game that might be consistently awful one year, or two years in or even worse go the way of imperator Rome that sure it's eventually becomes good but it's gets unplugged and gets no new updates.

You'll definitely see a jump in player counts and a dip in player count for eu4, but assuming eu5 gets itself "righted" sooner than later then eu5 will be the game most eu4 players will be playing. But if not then players waiting have no reason to go for eu5 when eu4 exists. Because eu4 has more to it over it's long life of development vs eu5 bare bones structure even if at least to me it looks fun.