To them living to make a difference is pointless because practically nothing survives Solar death, and nothing survives a maximizing of Universal entropy.
To them living for themselves is pointless because nothing brings sustained happiness. Solving problems, building things, etc. are just as much drugs as heroin. Once you've solved the problem you start seeking a new one. You can't be happy and satisfied with solving an important problem once.
You imply that the person would miss out and regret it, but they woudn't be missing out because they wouldn't be.
Suicidal people reading this and thinking I'm arguing in favour of it stop right there - I'm on a phone and can't write a full nuanced answer. But I think this is an important point which all the "permanent solution to a temporary problem" people don't really consider, and should, to see why its an inneffective exhirtation.