Years ago, the majority thought that Undertaker should have retired with the streak intact or, at worst, lose the streak to a 20-year-old, rising talent. In 2014, however, WWE pulled a swerve by having Brock Lesnar, a 36-year-old established star, end the streak.
Today, 3 years after Brock's win, I will explain precisely why Brock's ending the streak was the right call, as 3 years has been enough time for us to properly judge:
1). Brock was able to carry the momentum.
Upon ending the streak, Brock used his heat to evolve into an unstoppable demi-god, creating the charming "Suplex City" brand, squashing John Cena at Summerslam 2014, destroying Roman Reigns at Wrestlemania 31, and nearly killing Randy Orton at Summerslam 2016. He also worked a nice revenge storyline with Undertaker himself in 2015.
Those storylines, matches, and moments extend from 2014 - 2017. The significance is that the streak rub, which they gave to Lesnar, provided 3 years of excitement. The streak, therefore, was not wasted on Lesnar.
As a foil, I will point to the manner in which Kevin Nash broke Goldberg's legendary streak. When Nash ended the streak, WCW failed to run with it, as Nash didn't use the momentum to evolve his character and create moments for the fans. Instead, he participated in the fingerpoke of doom incident the night after Starrcade 1998, which nearly invalidated all momentum he gained.
Nash in WCW showed the way a streak rub can be wasted; Lesnar in WWE showed the way a streak rub can be productive.
2). Brock helped the future.
Perhaps the biggest criticism of Lesnar's ending the streak is that he was in his mid-30s and that the streak should have gone to some 20-year old stud. However, these criticisms fails on 2 counts.
Firstly, Brock was in his mid-30s, but his age doesn't entail that he is nearing retirement. After all, he wrestled full-time for just 2 years before leaving, and upon returning, he has worked part-time. Thus, Brock doesn't have much mileage on his body, the way Mick Foley, Steve Austin, and Shawn Michaels did; so he can viably go for years to come.
Secondly, Brock did help the future, even if he doesn't directly belong to it. By ending the streak, again, Brock gained significant heat and momentum. He thereafter used that momentum to feud with 29-year-old Roman Reigns and 28-year-old Seth Rollins in 2015. Those feuds with Reigns and Rollins helped create two mega-stars. Reigns and Rollins are main-eventers today because Brock ended the streak, in other words.
3). Undertaker needed the change
21 wins is plenty; he had already beaten several HOFers and future HOFers; he had already kicked out of 100s of chairshots and finisher moves; he had already won the belt multiple times at 'Mania; and he had already put on 5-star caliber matches at 'Mania.
What more could the guy do with an intact streak? The mileage was, reasonably, exhausted; it was time to change.
By ending the streak in 2014, Undertaker's next matches in 2015 and 2016 carried more suspense because fans speculated that he would lose and retire. His upcoming match with Reigns enjoys the same benefit.