Everywhere (both canon and non-canon sources) speak of the last dragonborn...but I still can't understand why he/she is the last.
Tiber Spetim was a DB, and all his successors has the blood of dragon hence considered as Dragonborn. So the dragonborn line should continue down from the LDB (or is it that they can't have children )
And as the Greybeards fear...if Alduin does return, then who'll stand up against him ? Paarthunax ? We can't have another DB since the Skyrim one was the last :|