John Cena -- I'll Never Quit WWE ... Even If Acting Takes Off

John Cena says he'll NEVER turn his massively ripped back on the WWE -- claiming he wants to work with the organization in some capacity FOR LIFE ... even after he's too old to get in the ring.

It's an interesting statement considering Cena's in one of the biggest movies in theaters this weekend, "Trainwreck" -- and his acting career is blowing up.

But don't count on Cena ditching Vince McMahon for Hollywood -- 'cause in NYC this week, he told us the org. is basically part of his DNA.

"I dont think I'll ever be done with the WWE," Cena said ... noting that if his body ever can't handle the rigors of the ring, he hopes to transition to either the announcer's table or something else behind the scenes.

BONUS -- Our photog Adam asked Cena for some squat advice on the street. Cena obliged ... and then shamed him appropriately.