That's why the time to end it (if it is to end) would be Undertaker's very last match before retirement. After he's gone, the only time the Streak will be mentioned "live" on WWE productions is when an announcer has occasion to refer to it. At that time, he'll say something like: "Remember the Undertaker's streak at Wrestlemania?" and the recollection will be a nod to an event of the past, which is perfectly acceptable.
The Streak exists.....the Streak ends. That's the beauty of athletic events: there are winners and losers, and in order for someone to win, someone else has to lose. If an achievement that existed comes to an end, it doesn't negate the former existence of the achievement. The only way that happens is if someone starts another Wrestlemania winning streak that eventually exceeds that of the Undertaker.
My preferred scenario would be to see the Streak end at the hands of the #1 performer in the company, John Cena......the implication being that only the top person in the organization would be capable of ending the longest running record.
Problem is, this is something that should be cheered (for the victor) as well as mourned (for the Undertaker). Somehow, WWE is going to have to find the way to turn the Cena-haters around so they'll be able to appreciate his achievement.....and that's gonna be a trick.
I do agree with your first two paragraphs wholeheartedly, even though as a Huge Taker fan, I am one who believes the Streak should remain unbroken forever.
As for Cena breaking it...well I am fond of Cena also, and I do think of the Current Full-Time, he is the only one with a legit chance of breaking it based on his current status. The likes of Bray and Reigns will have to be built first, to actually be considered in the future, as long as Taker can still go in the ring.
John Cena, however, can only go heel, if he beats the Streak. I argued as such late last year in another thread, and based on the reactions since then, that has been strengthened even more on the matter.
Cena beats the Streak, then there is no way to turn the Cena haters around, actually there will be whole lot more, especially when you consider that even today on a Part-Time schedule, the Undertaker still gets huge cheers when he appears, whilst Cena gets closer to a 50-50 in most arenas, ending the Undertaker's Streak and by extension his Career, would most likely move the Cena reactions to 0-100 in most arenas,lMO.
Cena breaking Taker in his last match can work, provided WWE has found their new Major Face going forward, as with a Family Friendly product, there would be no way Cena goes heel and still is marketed as he is now.