This'll never happen.
There is lots of demand for these through services. Off the top of my head, there's the demand from the large town of Kirkcaldy, the students and tourists using Leuchars for St Andrews, academics and professionals going on business to Dundee (a city with a rather poor airport), and the oil market in Aberdeen. Surely there's enough there to warrant through services to Newcastle, York and London?
And, believe me, the Express East services are rather well used. The HSTs help to provide some valuable relief. Whilst they could certainly be used more effectively (eg, by running in the existing Scotrail paths rather than supplementing Scotrail's service), they are well used trains and really should be maintained.
I've always wondered if it would be more cost effective to have these services North of Edinburgh operated by Scotrail using the same stock. So, at Edinburgh, East Coast crews leave the train and Scotrail crews board. I imagine it would save on the cost of having two separate crews for the same service.