The Undertaker vs Brock Lesnar is 2 years too late. However, even if you accept the Undertaker's weak physical structure and even weaker in-ring endurance and stamina as compared to Brock who can still be said to be in his "prime", the WWE should've done much better with the feud. Instead, what we've gotten is a watered down Brock Lesnar fearing the lame deadman schtick, which once again has been done too many times. When the caskets and druids appeared for Kamala, he was supposed to be creeped out. So was Yokuzuna in 1994. It was organic and timely in 1992. The line between kayfabe and reality was wider and suspension of disbelief a lot more prevalent. How many believe in this age of google and twitter that there's this dude named The Undertaker who sleeps in a graveyard, rises back every time, and causes fear in his opponent's hearts by his mere presence? Well, not when they look at Uncle Fester anyway.
But if you notice the timeline, his feuds with DX/Shawn in 1997 were rendered lame because even with Caskets appearing, Shawn and HHH would laugh their ass off which basically is an offence to storyline/kayfabe. But guess what? It worked. People still cheered for The Undertaker to come out and beat the living hell out of Shawn Michaels. But then, The Undertaker was a strong and imposing presence in 1997 and 1998. So was it with his feud with Austin. So much realism and so little scope for non-sensical druids/caskets crap. I bet Austin would've stood up on the casket besides the ring, stomped it, and said "If you think this casket contains a pile of poop give me a HELL YEAH" and they'd have Hell Yeah'd.
Did they even recall and mention their past feuds? I don't think so. Have they posed Brock Lesnar as even a credible threat, leave aside an inevitable person to end the streak? I don't think so. The only thing they've done so far is repeat. It's the Undertaker who should be wearing the Eat Sleep Conquer Repeat T-shirt because at the end of the day, Mark Callaway hasn't done a single novel thing in years and yet people are so dumb that they shower arenas with awe and cheer despite his apparently physical limitations. Thus, he eats , sleeps, when Wrestlemania comes around, Conquers another big paycheck, and then repeats.
I thoroughly remember all their history you've mentioned, but the sad fact is, it's the other way around now. Can Brock beat Taker and end the streak? What's the last time he F-5'ed him or seemed to scare the shit of people because their beloved Undertaker and his streak are in danger?