Dialects do not just disappear or die. Like other elements of culture, they shift and change with time and are affected by many factors. When people move to towns and cities and then out again, a process of ‘dialect levelling’ can occur.
In other words, dialects can sometimes become less distinct from each other. Linguists refer to this as supralocalisation, and this has happened to some extent in the South-East because of migration in and out of London, leading to the new south-eastern variety known as Estuary English. But this isn’t dialect death; it’s more a process of merging and re-alignment. In other words, dialects can become more like their neighbours in some ways over time, but they don’t disappear altogether, and new ones can form too.
One fascinating example of a new dialect is Multicultural London English (MLE). MLE formed over the last 50 years as a result of contact between different languages and dialects in London’s culturally diverse neighbourhoods – particularly Cockney and varieties of English from the Caribbean.