They are the last two msycital characters in a day and age that blurs KAYFABE and reality, the last two true gimmicks wwe created and fans accept even after Taker rode a bike and Kane unmasked. They they are the the WWE version of Kane and Abel personifiied, Kames aggression theme co-signs that. They have never had their brotherhood called a sham ala Edge and Christian. They are the two single most destructive forces in the WWEs history when they team up. They are loved by the fans. Their stories are also intertwined and will be until they retire.
To me at the end of the day the fact is they should have a third WM encounter and both hang up their boots after it. To me only Kane can end the streak and in doing so, they can claim his character has found peace. I also think that there should be a huge theatrical pyro, lighting, rain, and etc showing after that encounter as both disappear forever in the darkness and fire finally at peace.
From a business side though casuals love them. The facts their little feud over the WHC got very good ratings, unlike Chirstian and Orton on top. They also did good buys headlining ppvs. Fans just love them whether they face one another or form BOD again, especially when they reunite as they have been together so rarely since 2001 as the wwe never over does their as team unlike DX when they reformed
Couldn't have said it better myself. I've always thrown around the idea of them having Undertaker bring in a successor and then WWE having said successor take him out, but that would take years of build-up.