Of course it could have been won--and with far less loss of life on our side. But a winning plan would have required the use of overwhelming, unrelenting, and devastating force from Day One, force sufficient to utterly break the morale and will of the enemy. That is best done early, in an all-out sprint, before the enemy finds its legs. If you build up to it slowly, you only allow the enemy to harden and become inured to death and hardship. You make your enemy into a marathoner.
The north Vietnamese were not superhuman by any stretch of the imagination. They could have been defeated. We had the ability to defeat them, but neither the will nor the plan to bring it about.
Also the ally. The ARVN troops had low morale and many of their officers were corrupt. If the ARVN had the tenacity of the NVA then, yes, the war could have been won. Bottom line, our ally didn't have the willpower that the NVA had.