You mean the Hero of Time's bloodline ceases to exist. I really don't see the big deal there, as most of them are unrelated anyway. In the Decline Timeline he is outright killed, there is a short period of peace and then the formerly innocent people of Hyrule go to war against each other over the Triforce and the Gorons and Zoras move to Holodrum and Labrynna, and Hyrule slowly starts to diminish. When that happens, nearly the entire Hylian race goes extinct and 'Hyrule' goes on the verge of losing its name. The Decline Timeline is not only depressing, but we still haven't seen it recover. The last we saw of it, Ganon's minions are scattered everywhere and are out to kill the latest Link. Hyrule itself has moved, leaving the old kingdom in ruin.

At least we can say that the Adult Timeline is much happier world in its latest appearance. The land is vibrant, the technology is advancing and it is being protected by incarnations of Link and Zelda who are much more capable than many of their predecessors because they make a great team.