With the merger of Carlton and Granada many many years ago the newly merged company had to honour the existing trading agreements with advertisers meaning that they could not increase prices by that method.
As ITVx increases and more of these advertisers move more of their ad spend over they are subjected to new and higher prices ( in some cases double that from the terrestrial channels). Add on any subscription and it means that this part of ITV may be undervalued.
Taking away money and viewing from the terrestrial channels is not an issue. The base price on which deals are set is based on money divided by viewing levels. If these fall by the same percentage levels as an example then prices
I bought airtime from all the UK channels for MediaCom for 30 years.