I doubt there'll be many regrets. We did get a few memorable moments out of it - Rock's first return promo announcing himself as the guest host of WM27, Rock's History promos (the first two anyway, the third was just ok), Rock getting a big return victory in his home-state and winning the WWE Title once last time, the first Rock/CM Punk promo, The Shield's beat down of The Rock and Punk's Harley Race-like promo as he sit atop the booth near the nose-bleed seats, The Rock concert, Rock making fun of Vickie Guerrerro, etc. It wasn't as good as I had hoped, mainly because I had no interest in seeing Rock lose to the guy he was inevitably going to lose to (Cena) but it wasn't all terrible either. And the WWE definitely got great business out of it.
I think the 'issue' if you want to call it one is that the Cena/Rock match would have been better if it had been done 4-5 years ago. Say Wrestlemania 24 for example, which is ironic considering that was the year the Cena/Rock "feud" began. Rock could have defeated John Cena at Wrestlemania 24 and then a year later, he wins the WWE Title in early 2009, making his history with that belt ten years in the making (a full circle moment - his first WWE Title win being late 1998 and his last being early 2009.) Then at the big 25th anniversary Wrestlemania, Cena gets his win over The Rock and perhaps the victory seems a lot more meaningful then. I know he was already four years in as a top guy at that point, but so was Hogan when he defeated Andre at Wrestlemania 3, and that was the pinnacle of Hulkamania and the 80's boom period.
(I really don't think the idea was for Cena to get 'more over' with his win over The Rock though, it was just a dream match for many and a traditional "passing the torch" moment even though Cena had been carrying the torch for nearly a decade already. No different than Rock beating Hogan, though.)
Afterwards, The Rock could have still come back again in these past couple of years but feuded with someone else. A match with Brock Lesnar obviously, but also an angle with a newer star where that person could have gotten the ultimate rub. Of course, that leads to another problem that persists today, which is that no new star has been built up to a level where a match with The Rock would be considered must-see by people.