No, he didn't die for nothing. The one thing he really wanted was for the people he cared about--Mikasa, Armin, etc--to live long and happily, without the threat of being eaten by titans or enslaved by Marley, etc, outside of walls.
Well, they lived outside of walls, built a civilization, grew old. There were childfren and grandchildren (that shot of them bringing flower to Eren's grave showed multi-generations).
His loved ones got a full life in a blossoming world.
After they died, wars started up again, but normal wars, humans against humans, as always was and always will be. NOt Titan wars.
But he got his wish for them to live happily and long and freely.

Lol someone please explain...so Eren died for nothing? Like yes there will be always war but the Titans will come back or na?? IM HAPPY MIKASA IS ATLEAST MARRIED AND HAS A KID ╮( ̄▽ ̄)╭