It's always been like that. You did not want to skip out on paying your knights, and you really, really did not want to skip out on paying your mercenary soldiers if you were a feudal lord.
Yes and no. If the knights were poor enough, they couldn't field forces without help from their lieges, which kinda left them screwed. Happened in the later Tokugawa period in Japan and in various places in Europe as the Crown consolidated power. That was kind of the death knell of the feudal system in both cases -when retainers had so little power vis-a-vis the Crown that they became relegated to bureaucrats and administrators instead of actual military commanders.
And you could get away sometimes with not paying mercs as long as you let them pillage the shit out of whoever you were fighting.
If you don't pay local soliders, they might consider trying to overthrow you, but it isn't very likely if you pay your mercs or knights. Plus, they still live in the area, and another lord would eventually be given claim of the land even if they did win. So that isn't too much of a problem.
If you don't pay your knights, and they aren't so poor as to need you to support them, then they stage a coup and kill you. Given the number of entries in the Code of Chivalry about loyalty, they would probably try to do so with as little fuss as possible so they don't die themselves when someone else is appointed liege lord of the area.
If you don't pay your
mercenaries? You are so very screwed. They'll probably offer their employment to the enemy. While they're still in your city if defending, or your war encampment if attacking. Thus, they'll raze the area while already behind your defences, and burn you alive or somthing equally horrible. If you even have mercenaries in the first place, then your local soliders and knights obviously aren't enough on their own, and so the mercenaries stand a good chance of winning. It's fully possible that your mercenaries wouldn't have even had a chance to do any pillaging, much less be allowed to do so with what little spoils a weak lord would get.