How relevant is "efficiency" when we have an unrestricted loading gauge and 400m trains at our disposal?
The capacity of HS2 is so overwhelming compared to the classic railway that it renders inefficiency somewhat irrelevant in the near and medium term.
The Tokaido Shinkansen still manages 12 trains per hour even with at least three stopping patterns.
Capacity is no use if you have no way to use it, which is the situation HS2 finds itself in now.
As for non-stoppers, how many would there rationally be if the train goes through Birmingham?
The problem HS2 was trying to resolve was the lack of rail capacity on several lines all at once, whilst a new HS East would be great (and would probably be justifiable, especially if you consider the rail capacity heading to the Southwest, with a population of Devon and Cornwall if 1.4 million, and compare to that heading to our major cities in the North and how the populations compare) there was always going to be the risk that Leeds, York, Newcastle and the like would complain that they weren't seeing improvements, but they were all focused on the west of the country.
Arguably the capacity created on HS2 by removing the HS2 Eastern services from London could have been replaced with more services from Birmingham heading to the Eastern Arm and more trains from London heading northwestwards.
With 18tph (let's say 16 to allow for a bit of spare capacity) you could have had 5tph to Manchester, 5tph to Birmingham, 3tph to Liverpool and 3tph to Scotland. However, even then, in time we could have found that wasn't enough and that the widening of HS2 to 4 tracks could have been needed.
Between 2009 and 2019 Coventry went from 2.4 million to 8.2 million. That (at 342 passengers for every 100 in 2009) far outstripped the London West Midlands/Scotland/ Northwest growth (at about 170 to 175 passengers for every 100 in 2009), which in turn bet the expected growth rate (128 passengers for every 100).
Whist we are still a long way short of the previous passenger numbers (170) the amount we are short from the HS2 model numbers for 2024 (145) isn't that as far away from the 2022/23 numbers (128 for London Northwest) and given the was a significant number of problems and strikes which was suppressing rail use it wouldn't be a surprise to see passenger numbers reach close to the HS2 model numbers within a year or two of the strikes being resolved (if not exceed it).
Especially given that Long Distances rail use grew by 11% from September 2022 to September and Avanti saw nearly 25% growth over the same time period.