I agree with there being no way on Earth that Vince would have let Lesnar go over Taker at WM, thus breaking the streak. Vince likes loyal guys, hence one of the main reasons why Undertakers "streak" exists in the first place. By contrast, Lesnar is probably one of the most "disloyal" guys McMahon has dealt with in recent times. Brock left WWE at the pinnacle of his career for more "legitimate" pastures and had equal responsibility in ruining what should've been at least a passable bout with Golderberg at WM prior.
As for the streak in general, I'm a firm believer that it has to end sometime. Whether that means Undertaker retiring with the streak intact or somebody finally beating him, there has to be a grand finale of sorts. Truthfully I'd rather see the latter happen before the former and actually, I wouldn't be surprised if Taker (and probably Vince) felt the same way.
Undertaker is an old-school guy who knows how the business works. Like Shawn Michaels and Ric Flair before him (not to mention countless others), he knows that when you go out, you go out on your back. If you really are a loyal, team player, then you try to "make" the guy that the company has essentially set up to replace you or fill your spot. Again, Taker knows that by "going out on top" that this won't happen. He also knows that there is no better way to "make" a guy than by having said guy break his WM win streak.
The streak is there for two reasons. One, to make Undertaker even more of an attraction and phenom by giving him something that no one else on the current roster has. Second, it's to build something up so huge that when somebody does finally break through and find a way to win, that person is catapulted into superstardom pretty much immediately.
Providing something entertaining now while preparing for the future is pro-wrestling booking 101. Taker knows this, and Vince certainly knows this. SOMEONE will break the streak eventually, it's really only a question of who and when.