If Lesnar was full-time, 100% healthy and loved everything about working as a wrestler in the WWE and would be doing it for the next 5-10 years, I would believe him ending the streak. Even then as far as destructive kayfabe power goes, Lesnar's probably legitimately the strongest guy they have full or part-time to face Taker, even with that loss to Cena which does hurt him. I'm kinda hoping though Lesnar gets his win back over Cena at SVS. Right now for SVS I'm hoping for:
WWE Title- Undertaker vs. CM Punk (Punk winning)
Brock Lesnar vs. John Cena (Lesnar winning)
World Title- Sheamus vs. Orton (Don't care who wins)
All those are awful ideas
For starters, the streak is too big. Its like commissioner Gordon described Batman in The Dark Knight Rises. The streak could have ended in WM21, that was the appropriate time. Or some time very shortly after to some huge future star. But now, its almost bigger than Undertaker himself. Punk can beat Undertaker, but not at Wrestlemania. At this point nobody can. If anything, the matches vs Shawn, Edge, Batista and Triple H just proved its more impossible than ever.
At this point, nobody can end the streak. Its just too big. Too much legacy, too much emotion, too much importance. Its the single most valuable accomplishment in WWE history at this point because of the perfect booking it got
If the WWE could book the titles like they book the streak, we might finally have a decent promotion going again
And Cena losing to Lesnar would be beyond awful. Lesnar does not need his win back, for Gods sake the man beat the fuck out of Cena and he isnt even a full time wrestler. And if he beat Cena, thats Cena's 3rd loss in a row at Wrestlemania. If he loses to Lesnar, it will be the final nail in Cena's credibility coffin. Thats it, hes done. He'll either need to be turned mega heel or just bumped down the card because every loss he takes will cripple him from that point on and even the kids will stop seeing him as a threat.
The last time Lesnar beat everyone and left the company, it left it in a shit state that some could argue we still havent recovered from. As far as Im concerned, Lesnar is here to make Vince money, hurt people, and put people over.