Linear TV is ageing, and the largest channels are ageing fastest. There is an ongoing double-whammy effect of a growing older population, and the loss of younger viewers to social media and SVOD services.
The PSBs are suffering more than most, especially the BBC channels. 31% of the population is aged 55+, but over 60% of viewing to BBC1 and BBC2 is by those aged 55+.
The trend can be halted, and even reversed to some degree. There is no inevitability to this ageing process, but it will take concerted efforts to fight it.