That key obstacle is manpower, which Ukraine does not have.

The problem is not blatantly admitted, but implied by numerous officials with inside knowledge of the war.  US politicians (Democrats and Neocons) have been pressuring Ukraine to lower the conscription age to 18-24 year old men, a move which the nation has tried to avoid.  Why?  Because the Ukrainians worry that if they do there will be no viable men left to start families and replenish the population after the war is over. 

That's not a very optimistic appraisal of the situation on the front lines. 

The height of Ukraine's successful push-back against Russian forces in 2022 just happened to coincide with the height of the foreign mercenary presence in the region, with tens-of-thousands of highly experienced contract soldiers from the US and Europe helping the Ukrainians counter the maneuver warfare tactics of the Russians.  However, when the Russian tactics changed to attrition, the mercenary pipeline suddenly slowed to a trickle. 

The mainstream media suggests that the reason the foreign fighters stopped showing up was because the "romance of the war" was gone.  It's more likely because western soldiers are rarely trained to fight under attrition warfare conditions, making death a far greater possibility.  The point is, Ukraine no longer has a pool of foreign fighters to fall back on and their recruitment efforts are bearing little fruit. 

Young men are conscripted or in some cases kidnapped by military police, thrown into vans, dropped off at training centers and then dumped on the front lines within a few weeks.  Manpower is the key to war, and it's the one thing Ukraine has not received from NATO governments.

But if one examines Zelensky's victory plan it's as if someone somewhere must have promised him a fresh supply of NATO troops.  In fact, Zelensky's only plan seems to be immediate NATO membership which would then, under NATO treaty, require the deployment of NATO troops to Ukraine for mutual defense.  In other words, the Ukrainian plan would facilitate WWIII.