There are different kinds of stability. One is whether the same language is spoken informally. Another is whether people are able understand earlier texts. I'm pretty sure texts from the 20th and 19th century are commonly read in High School, TV shows from 1960s onward are watched, etc. Having the past ever-present makes it something current informal language relates to.

Many, many people speaking English also creates an incentive for stability at the same time it creates variation.