While true in some cases, there are plenty of examples of countries or long wars where there was no war weariness.
Rome was at almost constant war defending the borders and acquiring territories. Their people loved them for it. They had huge celebrations in Rome with Days of feasting and free food after every victory. The mechanic isn't a bad idea but it certainly isn't the only way civs operated. America on the other hand can't be in a war for more then like a few months before everyone gets tired of it and wants the troops sent home. But that's western philosophy and democracy. It wasn't always this way.
And contrary to popular opinion today many soldiers used to be proud of being in their army and liked fighting. It was a different culture back them. War was a way to win glory and a name for themselves, to be granted titles and rewards by the King, and one day come home rich. War was in some ways what young men in America see entrepreneurship and business as. The reason was back them there wasn't class mobility. You couldn't become a noble or be honored without being recognized by those above you.
I really wish the Civ5 honor tree reflected this in the way it operates. That winning wars would cause celebrations and make your people happy like it did for Rome. With current Civ5 you are forced to burn most cities just to stay happy. There is no way to conquer big empires early without taking huge morale penalties to your troops from unhappiness. The name "honor" implies that your empire glories in war and victory and your young men want to fight.
If we are ever to have war weariness in civ again it has to reflect these cultural differences that Civ5 has done a good job outlining in social policies. War weariness magnitude should be a sum of your people's philosophies:
Base war weariness: 1
tradition: 0 (little effect)
honor: -1 (makes people more likely to like war)
liberty: 1 (more significant effect)
piety: -1,0, or +1 (depending on your faith you chose your people are either more pacifist, not affected, or want you to war for faith)
commerce: +1 (trading tends to make civs like each other more and war disrupts trade. Most mercantile civs didn't like war for this reason)
patronage: 0 (no effect as long as you aren't abusing CS, +2 if you are)
rationalism: +1 (education tends to make people less supportive of war)
freedom: +3
order: 0 (technically communism makes the people complain less but I think this is more because of the police state. we'll pretend its just socialism in the absence of anything like autocracy as civ allows for a lot of hybrids like liberty/order - would be like sweden)
autocracy: -3
Also there would need to be a difference between defensive and offensive wars for this not to be really annoying. Civ 5 leaders often won't even talk peace for 10 turns or so and some warmongers never let up for years. If you get war weariness fighting on defense that would just be trolly since there is no possible way you can make peace. Your people would basically result in your empire being destroyed.