Oh no - not another chance to go bashing decisions made over 50 years ago in a completely different era.
It all depends what is meant by 'strategic' - lines on a map or potential for business?
Based on the latter, and evidenced by the amount of traffic conveyed since re-opening, I would suggest that the closure of the line from Farringdon to Holborn Viaduct (Low Level) has been the greatest strategic loss (and since re-instated)
I would argue that "strategic" means some greater value as part of the wider network, over and above that relating to the areas actually served. So for example, whilst Bodmin Road - Padstow would have had excellent potential for business, as a branch line, its value would mainly be in terms of those areas.