I think they can hold on, but to what end?
South Vietnam would have been basically like Chiang's Taiwan: a corrupt and kleptocratic nationalist state without the benefit of an ocean to keep enemies away and without a strong leader to forestall constant coup-related instability.
US Airpower is a powerful thing, sure. But it can't fix inherent issues of poor governance, nor can it end a war in which South Vietnam is already infiltrated by the NVA. The Viet Cong were basically destroyed by the US, but the NVA could infiltrate the country and take up old Viet Cong positions almost at will until Nixon expanded the war, and after the US drawdown, they could become bolder.
I would have to disagree on that as far as South Vietnam being a second Taiwan, more a possible South Korea in the making. I don't what you've read for sources but most of the Orthodox historiographic literature does not touch on the later period of the war, even some of well more known books cover largely up till 68/69. I hope to god this isn't ranting.
By the early 70's The South was in a better military position then the North having more troops and better equipment, all they needed was sufficent air support, the North, in contrast, had greatly suffered from a combination of the Tet and Mini-Tet offensives being failures and a prolonged bombing campaign. Since Thieu had also pacified the hamlets with the aid of General Abrams, guerilla tactics would be hard to try to because there is no more of a local insurgency to use. The next NVA offensive would have been their last, they would either to succeed or give up.
Politically and domestically the South had been under President Thieu who had least ruled for
10 years, which was longer than Diem who lasted for 8 and much longer than the previous merry-go-rounds of military leaders who lasted a month or two. Also, Thieu had actually managed to bring security to the countryside and wasn't as stupidly divisive as Diem was. So The South had somewhat decent governance.
As for the North, they cycled between the Pro-Soviet North-First and Pro-Chinese South-First, which also meant the Sino-Soviet Split kept the government divided and fighting with itself. Also both the Soviets and Chinese sent military aid when the North needed soldiers, and to top if the Chinese sabotaged Soviet aid and lied about its effectiveness. The Chinese government then diplomatically was not too different today, basically, if you do something they don't like expect to hear a lot of angry and some persona abuse. Even with someone Le Duan at the head of the North who was a South-Firster and pro-Chinese, when he called the Soviet Union a second fatherland, that earned no end and criticism that he angrily stormed off.
That said if Nixon does keep all possible measures save ground troops and there still is the rapprochement with China, it would only hasten the North's full embracing of the Soviets over the Chinese, and eventually isolation at least within the second world with Albania abandoning China over its rapprochement with the U.S.