My experience is more with the ECML than with the WCML.
The WCML engineering access rules give the appearance of being written by, and for the sole convenience of, the civil engineers. It's a stark contrast with other mainlines out of London, which generally have fast services until the last train of the day around 22:00-23:00
With regard to the ECML this is not correct. For example, tonight the 2300 York and 2333 Leeds are both retimed to depart earlier, at 2245 and 2315, and are diverted via Hertford North. They arrive at York at 0130 and Leeds at 0217. Tonight the last down ECML train via Welwyn is the 2200 Newcastle.
Well whatever is being done, it is rarely visible whilst trundling along the slow lines!
Midweek night possessions are important for overhead line maintenance. That has long set up times because of the need to get isolations and move equipment into place. In particular, frequent access is essential for adjusting tensions to cope with seasonal temperature variations. Not doing this risks more dewirements. For most of my commuting days there was a regular six weekly cycle of midweek night possessions each covering a different part of the route out of Kings Cross.
I'm old enough to remember the days before midweek night possessions. In those days overhead line maintenance was done on Sunday, usually lasting until about 1600, and using working methods that would not be permitted now. That still involved longer journey times with things like attaching/detaching diesel traction or single line working through work sites.
Increased Sunday travel drove the switch to midweek night possessions. Two important factors here were relaxation of Sunday trading laws and Network SouthEast promotion of weekend leisure travel, especially to London.
It is also important to remember the impact of railway shift patterns. Starting the work as soon as possible after 2200, and finishing before 0600, limits the impact of midweek night engineering work mostly to people doing night shift.