Nonsense. Housings can always get dirt in them, is likely the single biggest reason for deteriorating shifting performance, mt. bikes more than road, but it happens to both. Its usually the first thing recommended as a fix when a newbie on here complains about poor shifting performance. Mechanical is also a slow process, you don't realize sometimes that your shifting sucks as it happens slowly over time. Then you switch housing and cables and are like "I should have done that a while ago".