I can't see the Rock losing at Summerslam though, especially if he loses at Mania. You begin to diminish the Rock's value if he loses every time he gets into the ring. Punk vs. Rock would be fine, but the Rock begins to take damage if he loses.
Value to whom? Dwayne Johnson has no intentions of becoming a full time Wrestler ever again. As long as he gets his money, what does he care? Is he going to not be inducted into the WWE Hall of Fame because of those losses? Regardless of if he wins or loses, he will still be the Rock.
It's not like Disney is going to stop offering him roles in lame children's movies because he lost two PPV matches. Further, if he wanted to return to the ring full time, do you really think those two PPV losses would really matter one iota? Is Vince going to offer him less money in a full time contract because he laid down twice? Hell no. Is he going to be thrust into the midcard because of those losses? Again, hell no. Hypothetically, if the Rock were to want to return to the WWE full time, he would be automatically put in the main event/title scene immediately, regardless if he wins or loses those matches. He would instantly be at the very top of the WWE hierarchy, period.
Essentially, I would argue that rather than losing value by jobbing to current WWE Wrestlers, the Rock's value would stay put (at about the highest level you can get), while the value of his opponents will actually increase. Isn't that the real goal? To use the Rock to elevate current talent, to improve the current product? The Rock is a part of the WWE's past, not it's present or future. His value is in elevating the current product, not in demonstrating that an Attitude Era guy who hasn't wrestled in 7 years can still beat the best full time wrestlers of today.
If he is only going to even marginally be with the WWE until Summerslam, and nothing past that, he HAS to lose, otherwise the current product ends up looking weak, and that's bad for business.
