Perhaps. There would also have been political costs incurred due to the overcrowding on the southern end of the WCML and that then being apparently a lower prio

Perhaps. There would also have been political costs incurred due to the overcrowding on the southern end of the WCML and that then being apparently a lower priority than providing a quicker link between two cities. And yes, I realise the XC trains around Birmingham (inc. to Newcastle) are frequently overcrowded, but there's a comparatively cheap solution to this: buy more trains.