According to evolution

isn't like when a body part is repeatedly unused, it might like go away, so do you think that someday male mammals will lose their breast tissue and their nipples?

It's a complicated answer. Things do rarely disappear, most often they just get reduced in size or capacity, most of the time the structure gets so reduced but remains for developmental reasons. We have a tail bone because all chordates have tails, because the tail is a template for some important early decisions such as what end is anterior or posterior. The specific reason is complicated but is known. The appendix for example is defined early then nearby tissues draw cells for their own growth, no appendix then our guts probably would develop smaller.

Nipples are perhaps a different case. Males have them because their mothers have to have them as do their daughters. How could males lose nipples? Well it cant get deleted because they have to carry the gene (few sexual dimorphism genes are on the sex chromosomes, most animals not having sex chromosomes in the way we do. The most that could happen is some genetic basis evolving that could repress nipple formation in males. But it would have to be a very specfic thing. The genetic basis couldn't for example screw up all chest anatomy. How would it evolve? There's no apparent selective advantage to not having nipples no disadvantage either. So its left mostly the same way as in females.

Nipples can be seen as a byproduct. There's many little quirks in evolution.