Phase 2B sees Leeds-Euston and Newcastle-Euston services, therefore no need to retain such services* into King's Cross instead.
Similarly, Sheffield-Euston services would take demand off current Sheffield-St Pancras services, so more space for those joining at Leicester.
*Well, some services would likely stay in some form, but stopping at more stations to pick up intermediate flows more as trains will no longer be full of longer distance passengers.
This in turns takes more Stevenage/Peterborough passengers off GTR, etc etc etc
That argument isn't very persuasive to those of us who are cynical about HS2.
It might deliver some benefit for end to end journeys at some unknown point in the future, 30 plus years away, for some unknown cost.
And it'll free up space for the London dormitory town region because by then we'll still be more concerned with piling ever more distant commuters into a city that can't cope with it rather than spending all this money trying to reverse excessive commuting.
Except we won't free up all the space cos we reluctantly accept it's reasonable to want to travel from Newcastle to Stevenage, say, on the nice straight line between the two rather than changing at Birmingham. So instead we'll have to have a wider variation of stopping patterns.
I'm afraid I'm completely unconvinced.