I'll be damned, you are asking a constructive question and feedback? Let's try nice mode as opposed to not saying anything one more time then

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Why the manpower change then? Please do not play the realism card or something, there's too much counter evidence. The fact is there's practically nothing to do outside of war, everything else is a simple click and done with minimal to no thought involved. War is the meat of the game whether people like to admit it or not.
As for truces they simply cannot be ignored until your position is overwhelming because truces and coalitions snowball. Breaking a truce once gives you AE and therefore more nations join the coalition and more nations you have truces with even if you win the war and constantly truce breaking is a drain on monarch points and unless you've got innovative and/or defender of the faith (which not everyone is eligible for), the WE is going to add up too.
I think another factor is that truce timers and coalitions make mini goals harder to reach for inconsistent and 'gamey' reasons which results in players attempting to get around these gamey reasons in ways that shouldn't work (e.g declaring war on the ally of somebody who is in a coalition against you for the purposes of avoiding their coalition).
Something that makes many games addictive (this one included) is mini goals but when you do things like prolonged truce times and coalitions which change the rules of how much a player can be rewarded for the same effort it both makes those addictive mini goals harder to achieve for silly game mechanic reasons as well as causing a lot of frustration which is what you see here when people moan about coalitions, truce timers and multiple war leader changes calling in ALL their allies creating world war 1 over something completely ridiculous whereas something much bigger such as a succession war over France (though don't get me started on stupid PUs with large nations... just don't) could get ignored by most of the world.