But surely this is the distinction between short-term and long-term branding. Your short-term brand responds to changes in audience/content, but your (more minimal) long-term brand stays the same and should form the basis of your short-term brand. In TV channel terms - the short-term brand would be ident/presentation packages and the long-term brand would be the colours and/or symbols that identify that ident package as being for that channel.
Up until the consolidation of the ITV franchises, each region had a distinctive brand they had cultivated and refreshed over the years.
ITV could have come up with a symbol or icon that would represent the main brand, and adapt it for each channel. But they chose not to. For good or for bad, they instead decided to focus on a stylised representation of the initials.
Any symbol they may have picked would be competing with all the previous franchise symbols, so that may be why they decided to move away from that approach.
And I disagree that this is a TV thing - plenty of TV channels have strong long-term brands despite changing content. BBC One has changed at least as much as ITV in the last 24 years, but has retained a number of key branding elements across several rebrands. And in the US, NBC and CBS are often referred to as the peacock or eye networks because of long-running symbols. Hell, even ITV has the clockface of Big Ben for its news programming - which has gone through, not just visual-style changes, but presentation-style and format changes over many decades. It's not impossible for a TV channel to have branding elements that can survive for decades - many manage it, but ITV seems to struggle to hold on to even a colour for more than a decade.
The BBC has had the convention of placing their Initials into shapes (usually blocks or squares). But it was never applied in a consistent way until the 90's The channels also had their own identities (BBC Two less so until the 90's) which were crude in the early days, and were refreshed and re-interpreted over the years. There was never a single visual symbol which was retained without change.
TV Branding has tended to come along with a fresh channel controller, or content refreshes. Occasionally it is to reflect a change of audience, or an expansion of services.
Channel 4 has taken a US like approach with a distinctive visual which, whilst depicted in fresh ways, retains a core design.
ITV is consistently ahead of Channel 4 in its audience share and advertising revenues, which would suggest it has the more successful brand approach (but its far more complex a subject to analyse). To my eye, the current ITV logo is distinctive and fits the image people have of the network. It is also a group of channels I never watch.
In fact, pretty much everything TV related I watch, comes from iPlayer and Netflix these days. So this may change my perception of the brands, and it may be a direction that the industry is moving in. Content over Channel.
If Content becomes the defining factor for branding channels, then a strong "Masterbrand" will matter more when content is viewed outside of the Broadcaster's streaming services.