The question this begs is, what did Spock hope to achieve with his red matter originally?
My personal theory is that the red matter had a temporal component.
If all the red matter accomplished was to turn the existing supernova remnant into a black hole, it doesn't stop the shock wave from continuing on it's path of destruction. The shock wave is already gone.
However, if red matter has a temporal property, then it would basically reverse all the effects of the original supernova. The shock wave would never have been created and the planet Romulas would be "un-destroyed."
First the star supernovas', which threatens Romulas. Only then does Spock promise to save the planet, after the shockwave is already on it's way, "consuming everything in its path."
After Romulas is destroyed, Spock shoots the red matter into the supernova remnant. I believe what happen next was one of those Star Trek "reset events." As a end result of Spock's actions, Romulas was fine.
This next is from oldSpock's mind meld with nuKirk.
One hundred twenty-nine years from now
A star will explode, and threaten to destroy the galaxy.
That is where I'm from, Jim. The future.
The star went supernova, consuming everything in its path.
I promised the Romulans that I would save their planet...
We outfitted our fastest ship.
Using red matter, I would create a black hole
Which would absorb the exploding star.
I was en route, when the unthinkable happened.
The supernova destroyed Romulus. I had little time.
I had to extract the red matter, and shoot it into the supernova.
As I began my return trip, I was intercepted.