You seem to make the assumption that large contributions should be made within the duration of a PhD program. In my view, a PhD program's goal is to train new r

You seem to make the assumption that large contributions should be made within the duration of a PhD program. In my view, a PhD program's goal is to train new researchers. If awesome breakthroughs are generated before the thesis, of course it's a big plus, but not the point... PhD graduates still have several decades of career ahead of them to paint their Gioccondas. The PhD is just to get them started, when they graduate they should be able to fly alone and generate independent research. There is the problem that research funding (also after the PhD) is associated to projects that only last for a few years, but that seems to be pretty much universal.