As monstrous as he is, Joffrey does make a interesting point in season 1, episode 3. By allowing all the houses to control their own individual armies who can defect, it's much harder to keep armies together. In the middle ages, people didn't really think in terms of nation and didn't really fight for their country. They did so for their own personal homes, families, feudal lord, or for religious reasons. The Long Night saw the Glovers once again defect, and in times like this, we need the whole nation working together even if we don't like each other.
That was one of the reasons why in real life, feudalism faded out and feudal lords became or were replaced with governors, or counts. So, think the same is going to happen in Westeros? Westeros may have earned a respite, but war, corruption, and new crisis's will eventually return. When that day comes, the Kingdom of the North and the 6 Kingdoms need to be much more united and ready. Think feudalism is on it's way out? Only your imagination decides that.