[Galadriel55]

Firstly, I think it's not as important to know what the heroes lost as it is to know what they risked. The decision to put something dear on the line is in this case, I believe, more telling than the actual outcome. Earendil risked his own life, his family, and to some extent the fate of Middle Earth - no big thing, right? - when he sailed into perilous, unpassable, unknown seas that no one has been able to penetrate before. The fact that he hasn't sailed to his doom is his luck, but he put everything on the line to do what he believed was the right thing. That ought to count. Frodo didn't sacrifice his sanity. He lost his sanity, but he didn't sacrifice it. He didn't really willingly give it up in the name of the Quest. That happened as a kind of side-effect. However, he risked a lot more than that, and so did Sam. When Sam realizes in Mordor that even if they get to Mount Doom, that is the end, and still goes on - I think that's a much more significant sacrifice and risk. I know it's a matter of semantics, but I believe it makes a difference.

Secondly, individual sacrifices of great importance are made during times of great peril. Such times - and sacrifices - occured in the middle of Ages as well, they are not solely present during the end of an Age. However, the changing of an Age always followed a great victory or defeat or a bit of both, a great change that involved individual and collective struggle. So the logical chain is not Turning of an Age --> Individual Sacrifice, but Perilous Times --> Individual Sacrifices, partially due to which --> The Wolrd is not annihilated, and --> A New Age Begins.

Lastly, returning to your actual question about the recurring device. I'm not much of a fan of the Second Age and do not know much of the back history behind the War of the Last Alliance and etc., but the First and Third Ages definitely involved one individual sort of taking the brunt onto himself at the very end. There are multiple people who make sacrifices, multiple people who do their share - even do a lion's share, but since the work is so big it can take a heck of a lot of lions - but there's one who makes the final shove to pull it through, putting everything he holds dear on the line.