#2 They're half brothers!
This rivalry started 20 years ago, and until now there still is no clear winner. Undertaker did defeat Kane at WrestleMania 14 and at Unforgiven in an inferno match, though Kane buried the Deadman several times and defeated him on three successive occasions to retain the World heavyweight championship in 2010.
After Kane buried The Undertaker, the Phenom didn't exact revenge against his brother. We saw them team up to defeat the Wyatt Family at Survivor Series in 2015 and they crossed paths at Smackdown 900, but we're yet to see the end of this 20-year rivalry of feuding and teaming up, and as time continues to tick by, age clearly isn't on their side.
Kane ending the Undertaker's streak should've been their final encounter and ended the rivalry between the pair. After that, the Undertaker should've finally hang up his boots and retire as the legendary figure he worked hard to establish.