You're right; all of the people who go to Mars will die.
Of course, all the people who stay here on Earth are ALSO going to die. And while it's quite likely that the ones who go to Mars are likely to die sooner than the ones who stay behind, that isn't quite so certain - and we'll learn lots of stuff about Mars, and about ourselves, in the attempt.
In his book "The Right Stuff", author Tom Wolfe noted that all of the streets at Edwards Air Force Base were named for dead test pilots. Getting out of bed and leaving home, and going into space, or out to sea, or even across town; these things can be dangerous. We can't attempt to avoid all risks and still remain human.