Undertaker going over is best for business because a loss doesn't hurt Brock Lesnar's "mystique", because his fans are going to buy tickets to see him no matter what. Yet another loss to Lesnar is absolutely devastating to Undertaker, because the ridiculous lopsidedness of this feud is overshadowing everything else Undertaker ever did. At this rate Undertaker is going to be remembered more for a dozen losses to Brock Lesnar than ANYTHING else, because they're building the storyline to emphatically state that The Undertaker CANNOT DEFEAT BROCK LESNAR. That destroys everything Undertaker ever did in his entire career. It makes him worthless and it makes everything he ever did worthless.
And why does my naming Bret Hart as one of the legends who went out with major wins instead of losses confuse you? He went out on a streak of three major victories.
You are completely overreacting. Not only that but you are actually underestimating Undertaker. You really think a 25 year career is only going to be remembered for one lost feud? Why? Why does losing to Lesnar take away from anything else he did? Shawn Michaels lost to Taker two years in a row at mania. Did that make the rest of his career worthless? Triple H lost to Taker three times at mania. Does that mean everything else he ever did is worthless? Why are the stakes so high for Taker here? So what if he doesn't beat Lesnar. I don't see how that nullifies any of his other achievements. Undertaker has a strong enough legacy that people will remember him fondly despite the losses to Lesnar. WWE will give him the Andre treatment and always speak highly of him long after his career is over.
Your statement about Bret confused me because his final match during his WWF run was probably the most talked about loss ever. I guess you're counting some special attraction wins ten years after his career ended and after he suffered a stroke which physically reduced him to a shell of his former self.
But if you want to count whatever someone's technical last match is then quit getting so worked up about Taker. Who says this is his last match? He could still win his last match. It doesn't have to be against Lesnar.