It still takes 3 - 5 years or more even for that incremental progress. It takes years to just catch up on the field! Do we expect PhD candidates to subsist on barely livable wages until they eventually publish a ground-breaking result? That kind of disincentive to even start a PhD would not be conducive at all to progress.

Yes, most PhD theses are scientific and commercial dead-ends (even more reason not to gate the degree on ground-breaking results!) but they do serve to cull the problem space, and that's exactly why we need more of them. In fact we should even provide some incentives to publish negative results in academia.