A number of newspapers have reported this week that the "th" sounds will die out by 2066 in British English (here's an article in the Daily Telegraph). How likely is this assertion?
Dental fricatives [θ] and [ð], spelled "th" in English and found in words like thin and this respectively, are very low incidence in languages of the world; they are found in fewer than 50 of the world's 6000-7000 spoken languages. In some cases, they only occur because of a phonological process. For example, [ð] appears in Castilian Spanish between two vowels, where it is an allophone of the phoneme /d/.
Dental fricatives are also late acquired in English - i.e., children start using them later than some other consonants. SLT Info, for example, explains that English-speaking children do not start using them until around four years of age, while some consonants, such as /p/, /b/, /m/ and /w/ (what's the common factor here?), are produced as linguistic sounds as early as the age of two.
Most new varieties of English around the world do not use dental fricatives. Hong Kong English speakers, for example, produce /θ/ as [f] (three sounds like free) and /ð/ as [d] (this sounds like diss).
There are also accents of British English which have been around for a very long time which do not use dental fricatives. Do any British readers of a certain age remember the Qualcast advert "It's a lot less bovver than a hover" (see around 48 seconds)? This works because accents such as Cockney, for example, have been substituting dental fricatives for other sounds for some time. /ð/ word initially is often produced as [d], and between two vowels as [v], as in this advert. /θ/, just like Hong Kong English, is produced by Cockney speakers as [f]; in fact, as a child learning Maths, when the new teacher arrived who was ethically Chinese and from Hong Kong in the 1970s, we all thought she was from London as she pronounced Maths as /mæfs/.
Producing /ð/ as [d] is known as stopping - i.e., the fricative is produced as a stop or plosive consonant - and producing /ð/ as [v] and /θ/ as [f] is known as fronting - i.e., the fricatives are produced further forward in the mouth, in this case, as labio-dental fricatives. These are both processes which are common in developing child language in English. Some varieties of English stop /θ/, so it is produced as [t] or similar; Southern Irish accents do this, as does Jamaican English.
Given that dental fricatives are very low incidence in languages in the world, late acquired, and often substituted in regional and global varieties of English, it is not really a surprise that they are predicted to die out at some point in the future. This might be down to multiculturalism, or it might simply be because they seem to be of less importance in international communication in English. Which is it? It might be difficult to decide.