5. HS2 will make journeys that are currently about equal timing and frequency by east and west coast much quicker by one or the other. For example, Edinburgh gains a fast train via the WCML to Curzon Street, and suddenly you don't need to serve Edinburgh to Birmingham journeys via the ECML.
At the moment though, XC serves a lot (probably a majority of journeys not via London) of journeys from Scotland, the North East and Yorkshire to Birmingham and the south west.
If you cut these back to York, suddenly you force people into changing at York. Those journeys that were direct now require a change, so you lose passengers. Yes, on paper your XC services are now less busy and have more space, but you're just replacing long distance passengers with shorter hops, and your train may as well be a semi-fast. That's not a great use for voyagers.
6. In addition, as well as forcing people to change, you force people into other trains. That's not terrible for Edinburgh, but pretty terrible for Newcastle. The Avanti west coast service suddenly becomes the only Scotland to Birmingham service, and becomes exponentially busier. Leeds passengers are forced onto the 1tph TPE service, which looks ready to be binned anyway. Newcastle loses all of its Birmingham services, although still keeps 2tph to Leeds.
All those passengers that previously had direct journeys are now making the change at York, or flying. Not a great look, but also bad for operational reasons. Lots of churn at York means greater dwell times, and the LNER services get much busier with intermediate passengers rather than premium paying London passengers.