it's amazing when you think how many would-be tsars and possible descendant lines just died - if you consider just the Rurikids starting with Ivan III:
Ivan III's grandson Dmitri died in jail where he was placed by his half-uncle Vasili (also his father, Ivan, original heir, died young)
The Staritsa line, previously mentioned (apparently Vladimir had many children killed by Ivan the Terrible)
Ivan III had another son Yuri who was not allowed to marry and died childless
Vasili III also had a son named Yuri (deaf mute, his son died young)
Ivan IV's son Ivan was killed by Ivan IV!
Ivan IV's son Dmitri died in epilepsy attack (popular version, other versions include murder and escape which is unlikely)
not a good track record