Well, the game was originally balanced so that an average player could not WC. But what's considered an average player on the forums today is miles ahead of what an average player is, and what an average player was when game was released. We all learned an enormous deal by watching vids, reading AARs and reading answers from better players. It's hard to balance that so that new players still enjoy the game.
Anyway I don't see why making WC less achievable would solve the issue you raise, or has anything to do with it being or not the final goal of the game. I've done my fair share of WC, I do that once in a while still, but that's not my final goal in most campaigns. I don't see why making WC harder would give me more fun when I don't aim for a WC.
So far, we could say WC can be boring because of the excessive minmax/micro, and that tall play is boring because of the lack of things to aim for that do not require blobbing. Making WC harder, if anything, will make it even more boring because of minmax/micro, while not helping at all with the issue of tall play lacking flavour.
I think the problem is that the game need something else to do that is not conquest and war, something interesting to do, that is.
At the moment, the only thing interesting in the game is war, everything else is an accessory, you trade to gain money to afford an army, you research to have a better army (or to gain more money for the army) and you spread your religion so you have less rebels that distract your army.
So you make wars and conquest other provinces and grow stronger, and not just because it is fun, but because if you don't grow strong someone else will and then it will attack you, and then you'll lose the game. it is literally conquest or die
But you arrive at a certain point when you are so big and strong than wars stop being interesting because they are not challenging anymore, you being so much stronger than everyone else, and because there is nothing else interesting to do you usually quit.
now, world conquests, I'm not saying they are easy, but I don't find them interesting, even in the AARs I've seen are the most boring part (and a part where I hope something go wrong for the player because at that point they look like they are bullying the other smaller nations, yeah, I know, it is silly), they seems more an exercise in micromanaging and min-maxing where you synchronize building claims and war declarations to use more efficiently the remaining time and release the right vassals and feed them province to optimize your monarch points, and so on. Challenging? certainly, but not my kind of challenge.
The game need something else interesting to do that is not war, and a way to gain power that is not conquest (there are some way at the moment but they are inefficient if confronted with war) so maybe you could do something else beside conquering everything until it lose interest (and even if you do it you could have something else to do..