I'm sure he'd still have a very good year left in him by 2019. I know he's had a lot of injuries, but by the same token there's a few less miles on the clock.
That said, by 2019, we'll have had the best 8 years out of possibly the best striker the premiership has ever had (I know that's subjective, but i wouldn't swap him for anyone). To have kept one of the very very top players in the world in Manchester for 8 years and have him leave amicably to play for his hometown club, nobody can complain about that. An extra year would be nice, but still, what we will have had from him is way beyond what I could have ever hoped.