For them to be alive at the point Alisha was killed in the last episode, superhoodie Simon had to have gone right back and saved them in the past like when it was shown on the past episodes. Superhoodie Simon had already saved them - the present doesn't exist unless he goes back. He has to. Even if it was possible to have just gone back to when Alisha was killed, they'd both still be stuck. With that power, he can't come back. So he'd still be in a different time hours behind the rest of them. He'd still never see them again.
I think it actually makes perfect sense now, remember superhoodie telling Alisha he had to sacrifice himself/die so that she could live but they'd still be together? And that it 'had to be this way'. The writers knew how it was going to go down all along. No idea how they'll bring back Simon - I really don't think they can!
Would have been such a good end to the series! Curtis can't do anything with his power, Kelly is loved up with Seth, Nathan's left, Alisha and Simon are trapped in an eternal circle.
Rudy said at the end "A Happy Ending", and all the old probation workers came back for a goodbye. They should stop while it's still good and I can remember it as a legendary, unique series.