One of the merits of my proposal to break cross country routes at Peterborough is that it further reduces contamination of Greater Anglia from the rest of the network.
I have to disagree though. You seem to be suggesting forgoing the XC routes in order to use their paths for a more intensive service between PBO and CBG/NRW. I agree that their should be more provision on those routes. However, you'd need a significant re-modelling of Ely junction which is already sort of being mentioned. Furthermore, EWR is proposing to four track the route south Cambridge in order to separate flows from the Hitchin Line/New EWR line on one set of tracks, and services coming southwards from WAML on the other. The EWR long term strategy is to electrify and double the line to Newmarket and extend Liverpool Street trains to terminate their to open up a bit of space in Cambridge station from those terminating tracks. Electrifying that route might only be justified with the electrification of the whole Ipswich to Ely/Cambridge line because of the freight flows. Same goes for Ely to Peterborough in terms of electrification. Further down the line, the Leicester area has been slated for re-modelling for a while.
I know all these projects are kind of nebulous are fighting it out in planning hell, but they are actually being pursued by the rail industry to at least some extent, so there may be quite a bit of capacity enhancement happening in this area. That capacity enhancement is absolutely required for what you're talking about anyway but could most likely also accommodate the Birmingham to Stansted route.
I also believe that the demand is low because the service is bad, and as a sometimes user of the Norwich-Liverpool train (as far as Nottingham or Sheffield), the best thing they can do to that in the short term is run longer trains.
And, isn't this like the whole rationale of EWR? How many people do Bedford to Cambridge right now? Basically no one because the route by train is terrible - but they've clearly proven that need enough to be able to propose building a new line. Journeys like Cambridge to Leicester or Norwich to Nottingham could see more demand if they are improved by higher capacity rolling stock and journey time reduction brought about by a range of infrastructure investments (Leicester remodelling, Ely junction remodelling, FtN electrification, Breckland line electrification (mentioned by Norfolk County Council) which are simultaneously important to many other operations. Especially the higher capacity stock because that might in theory bring down ticket prices (in a world where ROSCOS don't exist).