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.