The American people don't have patience for a long foreign war like that. That is reality, at least under a democracy. It was certainly the reality during the Vietnam Era. That was the point of this author's book - that TET did NOT destroy the US public's support for the war in Vietnam. That is the liberal fiction and narrative created afterwards.
As to Vietnam, another 3-5 years of direct US support in terms of money, equipment and airpower would have bought the south a lot of time. N.Vietnam's 1972 Easter Offensive basically failed, due to the use of US airpower. By that point, the troops on the ground were all ARVN, not US Army. By abandoning S. Vietnam in 1973, we ensured their, and our, defeat.
Maybe....but let's remember what happened at the bitter end when ARVN troops ran like scared rabbits to the helicopters. People can make all sorts of excuses, such as cutbacks in U.S. aid, but ultimately that can not explain away a lack of fighting grit.
Equally revealing of a lack of fighting grit was the failure ex-ARVN, or anyone else in SV, to launch an insurgency against the new communist regime. Please note the contrast between this failure and, for example, the Afghans who initially launched an insurgency with surplus British weapons from the turn of the twentieth century or the Filippinos who launched an insurgency against the U.S. in 1898 despite a near complete lack of weapons.