It could also be pointed out that the most successful wars of the Medieval (and, to an extent, ancient) period were 'blitzes' of a sort.
The Mongols being probably the most successful example of a 'blitz' ever. Sure it was a blitz that lasted about two hundred years, but to be fair, it isn't like Hitler had to do it all on horseback, without roads, airplanes, preserved food and repeating firearms.
Considering they managed to (rather briefly, true) control the largest empire in the history of the world, one can hardly fault them for taking a while to get it all done.

Of course, the point of 'overextension' comes in as well. Empires established by blitz tend to be unstable. Ask the Mongols. Or Alexander. That's why I tend to adopt a 'tortise and the hare' approach in MTW. Blitz like hell the first few turns, carve myself out a nice chunk of land, then build myself an infrastructure. Gives me a nice place to push from for my next blitz.

Anyway, it could also be pointed out that the majority of the wars you listed, while defensive, were fought in an offensive manner by the defenders. The Vietnamese, Americans and Afghans didn't sit in forts or cities, waiting to be besieged, they went out and kicked American/British/Russian ass until the other side got so pissed off that they threw up their hands and said, "Screw this, the media is raping us/India is nicer in the winter/there's no vodka here."
And, in the case of the Hundred Years War, the French didn't start winning UNTIL they started fighting offensively. And had help from God, if you believe Joan of Arc, although they managed to win the war even after they sold her out.