It's a very good piece about data. Too bad it has no...... data.
McNamara was one of the few people who knew the war was unwinnable very early on. He used his (waning) power within the administration to facilitate back door peace talks.
The war continued because (to their equal discredit) Johnson had seen too many Westerns and thought the war could be won with one final knockout blow. Ho Chi Minh thought the war could be won by using the peace talks as a (master class) fan dance of bad faith whilst building up for the Tet offensive for.... one final knockout blow.
A little reading would not go amiss.