The propulsion technology in itself hasn't changed much, so it is not like you put a new engine and suddenly it is 3x faster. The problem is not technology, it is money. almost all the speed comes from gravity assists (get close to a moon/planet and let it accelerate you as you get closer), so to squeeze extra, you need to pass closer to more planets, which requires more fuel for the adjustments. More fuel means the probe is heavier, harder to build, etc. Maybe miniaturization of other parts makes up for that extra weight, but still, to catch up with Voyager would take decades