As much fun as it is poking holes in mcmanusaur's logic, the big point is, and should be:
Does the current system make the game more fun or less fun than reasonable alternatives we might try?
A large portion of the board thinks the current system makes the game less fun, having watched my wife's EUIV interest plummet, I suspect that less hardcore players also are finding the current mechanisms less fun. Certainly, the current mechanisms are extremely opaque and hard for new players to get into.
Let's run with the last one for a moment. You fire up a game with the Ottomans in 1452. You take the mission and go to war with the ERE and take all their land (not terribly ahistorical), you get 41.5 AE base with everyone. You then, proceed like history to go after Serbia, Wallachia, and Moldavia. Maybe pulling in another 120 AE. This will likely start getting you coalitions. You, not knowing the system, will then figure well the Ottomans beat the coalition at Varna and later at Kossovo ... the answer is to beat the enemy again. So you send in the army against the Polish/Hungarian/Bosnia/Venetian/Lithuanian/Mameluke coalition ... and then it down a never ending spiral the game never tells you how to mitigate. Long before you can besiege Vienna, you are caught in unending coalition wars.
But send diplomats you say - fine where does a new player learn that which attitudes will join a coalition? Where does a new player learn how to control AI attitudes. Where does a new player learn how to deter a coalition? For new players this punitive mechanism is basically a "don't go war" message.
Now how fun is it for veteran players? Well I have yet to see anyone post "here's a fun way to win the coalition wars", so I'm guessing no one likes the mechanism itself. But maybe people like avoiding the mechanism, okay fine, if that is a fun game for you ... you can still play that way.
So what are we left with? Fears that tweaking the coalition system (as opposed to the vassal system or combat system) somehow has a unique and unwarranted risk that WC might become possible for perhaps 1% of players.
But maybe WC is so dangerous we should design to make it nigh unto impossible. Okay. Are there no other options than the current system (which impedes historical play of nations like Russia, the Mughals, Spain, England, Rev France, Austria, the OE, and the Manchu)? I think not. Suppose nations joined a coalition. Every war against the coalition doubled their morale, discipline, tactics & manpower. These bonuses decayed by 5% a year. You could let the normal alliance rules sit in place (separate peaces, nations can leave anytime their relations change, etc.) and quite obviously banging your head against the coalition with straight up conquest results in your Prussian hordes of doom eventually being overcome by Ethiopian spearmen. Clearly, we could arbitrarily give anti-human coalitions bonuses to a point where the player cannot beat the coalition. So we have options.
Now where to balance the options, that is a challenging question. I certainly want the AI to have a good shot at overcoming a large player empire and whacking it for huge territory, but I don't want coalitions to basically end the game for people who can't fight efficiently and I don't want them so easy I can just expand until 1650 and then go roll up the coalitions to WC. But this is a workable problem - something that could prevent easy WC (limiting it to .01% of players even) ... but not nuking historical play by making the first formation of a sustained coalition the end of historical conquest patterns.
Yeah, I think the phobia of WC is big distraction from core issues - the game should be fun for all levels of skill and as many play styles as possible and if some cheese let's you WC, nobody forces you to use the cheese. But there is a lot of room between "WC is easy" and "coalitions are hated by half the hard core player base, and possibly more of the casuals" - the folks arguing to make WC impossible aren't stupid, but I cannot fathom why they refuse to contribute suggestions to what might achieve their goals as well as people like me. Surely they have some thoughts on what might be able to prevent WC - but not upset so many players. Even if they believe folks like me are just shills for easy map painting, good faith suggestions that assume my goals are as stated will do a much better job of preventing WC than "lolz go play Total War".
We've seen this behavior before. A bunch of us noted that trade was insanely more lucrative than buildings could ever be, and that buildings were pretty much useless with trade cash + conquest other people said buildings were great. Rather than have a good discussion about how to better balance buildings vs light ships (along with production & tax vs trade), we had a lot of folks just claim "everything is fine" and that trade wasn't that good of an investment. So we ended up with a less balanced nerf to the trade system than it could have been. Likewise when 1.2 came out we had a bunch of people who loved the new combat mechanics, others thought they were ahistorical in the extreme and unfun (e.g. losing your entire army 4 days into a war when you had defensive terrain, 4 more regiments, and a better leader ... but lost the die rolls 0 to 9 and 1 to 8), again the folks who liked it as it was refused to say why they liked the status quo and what should be considered when making changes. So we got a hotfix that gives us multi-year late game battles.
I really just don't understand why the status quo defenders won't get into the details of what goals they really want, what possible changes they think might satisfy both sides stated objectives (and no, easy WC is no one's stated objective), and what would make suggestions from the "other side" work better. That would be far more productive and historically is more likely to get you what you want.
//end rant
I suppose I should note that Vishaing has not always agreed with me about what are problems and I think he wants an EU IV where WC is outright impossible as a design goal ... but he does try to refine my suggestions (for which I am most grateful as they at minimum help me think about alternatives better).