Budget is the key factor. On an almost daily basis, I receive Quora questions asking “why we don’t do Mickey Mouse I retired there’s a nap for that shirt in space” type questions where the answer is almost always – because we haven’t been funded to do that. Budget defines everything for space missions and that means that everything is tested and built to meet the requirements plus a safety factor and no more. That doesn’t mean everything suddenly breaks at that chronological threshold. It simply means we haven’t put extra effort (cost) into guaranteeing it will last longer.