Luke was the last hope... even if Yoda still considers Leia to be another possible hope. He doesn't consider either Ahsoka, Kanan nor Ezra as a hope to stop the Empire... and only considering Luke and Leia because they are Anakin's kids to succeed seems like such a cop-out, when others had trained for much longer and would at that time be more of a threat to the Empire than Luke - which to me means they are dead at the time Yoda starts training Luke.
It is not a cop-out. Annakin is known as one of the most powerful jedi and general of the clone wars. Then he goes dark side which gives him a power jump. No one can beat the dude. Anyone that tries dies. You think Luke beat him because he was more powerful than Vader? He beats him because he is Vader's son and once Vader finds out he has a son cracks in his dark side armor start too show up.