Dear Sechel,
The litvishe yeshivos of pre war Europe were mostly shaven. This wasn’t a leniency. They knew their stuff. They disagreed with the Chofetz Chaim’s psak. That machine is now obsolete, so it’s not even a question anymore. But anyways, the last hundred years has yeilded more proofs against the Chofetz Chim’s position, than for it.
Clean working boys with good fathers go on the market around twenty. You can’t learn the same when you are married. They really should push off it even later.
Many Kollelleit finish shas or at least most of it, before they turn thirty. They just don’t advertise it, because they do not feel like they are ready to discuss any part of shas.
The not finishing the mesechta started fifty years ago. It is because there are so many boys going to yeshiva that aren’t pushing themselves hard enough to become gaonim. So, lack of productivity has become the norm in the last generation.
The percentage of yeshivaliet that learned the whole Nefesh Hachaim has been going up steadily. But I doubt it has reached even a 35%. Yeshiva guys need to learn non stop gemara more than anything else. Otherwise the yeshivos will crumble.