as john said, this future has changed.
just like john having knowledge of future changes it course, the computer/terminators that was sent back previously might have also altered the future, by planting vital information in the past perhaps?
thats the selfl- made up reason i took for why they were hell bent on capturing kyle reese when john connor wasnt the true leader yet, how they can have facial recognition of kyle. etc etc.
anyhow everyone else highlighted the flaws of the film so thats that.

im gonna highlight on a positive of the film;
'2nd chances'?

marcus the criminal was given a 2nd chance.
he was born again with a 2nd chance to live life.
trying to find redemption for his past life, he decides to embrace humanity and help the resistance.
in hopes of finding redemption and self-forgiveness, he willingly gives up his heart for connor. self-sacrifice. so connor now has a 2nd chance to be the true leader of resistance, and marcus can try to die in peace.
a villain becomes a hero/saviour.
unfortunately, this triumph of humanity was overshadowed in the film or the audience didnt fully feel the impact.
dats because it wasnt delivered/executed well enough too. it could have been memorable but this movie will be about the action sequences. also, this movie doesnt have many memorable moments apart from the best lines shown in trailers.

i think the theme '2nd chances' is just McG's way of asking audiences to give him a 2nd chance after his charlie's angels work.

on a sidenote; for the humour part; here's some made up stuff.
i bet marcus couldnt care less for living in this hellish world. so when the idea came to him to donate his heart to save the predicted hero connor, he probly thought; 'meh, why not?'