1) Because loan system was too complex to make it understandable to the AI, so was exploitable. 2) You misunderstood it. You can totally make peace under 75 warscore. You just need the other AI to actually want to. The rule is "at 75 warscore, the AI is forced to accept capitulation", which has as a byproduct that at 75 warscore, you can force the AI into peace even if it does not want to.