if it ain't broke don't fix it
Don't meddle with something that's functioning adequately. For example, So long as they like our proposal let's not change it; if it ain't broke don't fix it. This folksy and deliberately ungrammatical expression dates from the mid-1900s. For a synonym, see leave well enough alone.
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.