If no D-mail, in the future will have, he spent 14 years to figure out how to save Kurisu. Do you know why reading steiner didn't active after he sends it? Basically, no one knows about it, if you watch closely, you will see Suzuha open the D-mail video for him, that's why he has to save Suzuha and live with Mayuri in 18000BC to prevent reading steiner kick in and overwritten his memory in Steins Gate WL and damage it.
P/S: Don't judge, i'm just 14 years old