I agree with you. There is no guarantee that Ukraine will be another South Korea.
But the Polish-Jewish group in power is promising this idea to the Ukrainians and has managed to convince them to fight against Russia successfully.
But speaking of possibilities, the demographics of all countries in Europe and Asia are equally terrible, including in China. Despite this, Poland, Estonia and Lithuania are all growing to become near-developed countries already. China, despite poor demographics, is also growing and competing.
A country no longer needs to have a fertility rate of 6+ to become rich.
After the war, my prediction is that Ukraine will be divided into three parts:
1 - the Donbass strip will remain with Russia.
2 - the center becomes a fake "neutral" zone occupied by UN and NATO peacekeepers, just like Palestine/West Bank and with the male population devastated.
3 - the west is the real Polish-Catholic-Jewish Ukraine, with relatively good demographics and preferential access to US and European politics and investments.
That's why I think Russia should do everything to win while possible and not imitate Iran against Israel. A stalemate is dangerous, a defeat even worse.
Poland was the crown jewel of the Warsaw Pact outside the USSR, on par in terms of importance with DDR. They were more mechanized than China in WW2. There was nothing wrong with Poland economically, what holds them back is their bottom tier diplomacy where they somehow always manage to piss off much stronger powers.
It isn't about the demographics alone, it is the entire world environment.
In the 1950s to 1980s environment, the rising tide of petroleum exploitation and electronics was a tide raising all boats. The US was a massive economic anchor that pulled up those who clung to them. And both industry and resources were almost all under US or allied control.
Today the world is in an era of resource decline. China is the premier industrial power, Russia is the premier resource power. The west doesn't have the capability to pull Ukraine up. Paper money isn't capability.